| Type | Filed | Filer | Subject | Shares Owned | Percent Owned | |
|---|---|---|---|---|---|---|
| SC 13G/A | 2012-02-08 | VANGUARD GROUP INC | NORTHWESTERN CORP | 2,304,450 | 6.4% | EDGAR |
| SC 13G/A | 2012-02-08 | ARTISAN PARTNERS HOLDINGS LP | NORTHWESTERN CORP | 0 | 0.0% | EDGAR |
| SC 13G/A | 2011-02-14 | MUNDER CAPITAL MANAGEMENT | NORTHWESTERN CORP | 2,579,615 | 7.1% | EDGAR |
| SC 13G | 2011-02-10 | VANGUARD GROUP INC | NORTHWESTERN CORP | 1,960,257 | 5.4% | EDGAR |
| SC 13G | 2011-02-10 | ARTISAN PARTNERS HOLDINGS LP | NORTHWESTERN CORP | 2,064,648 | 5.7% | EDGAR |
| SC 13G | 2011-02-09 | PRICE T ROWE ASSOCIATES INC /MD/ | NORTHWESTERN CORP | 2,168,840 | 5.9% | EDGAR |
| SC 13G/A | 2011-02-02 | BlackRock Inc. | NORTHWESTERN CORP | 3,025,314 | 8.4% | EDGAR |
| SC 13G/A | 2010-02-16 | ZIMMER LUCAS CAPITAL LLC | NORTHWESTERN CORP | 0 | 0.0% | EDGAR |
| SC 13G/A | 2010-02-11 | MUNDER CAPITAL MANAGEMENT | NORTHWESTERN CORP | 3,050,916 | 8.5% | EDGAR |
| SC 13G | 2010-01-29 | BlackRock Inc. | NORTHWESTERN CORP | 1,924,456 | 5.4% | EDGAR |
| SC 13G/A | 2009-02-17 | ZIMMER LUCAS CAPITAL LLC | NORTHWESTERN CORP | 1,709,724 | 4.8% | EDGAR |
| SC 13G/A | 2009-02-17 | RAMIUS LLC | NORTHWESTERN CORP | 0 | 0.0% | EDGAR |
| SC 13G | 2009-02-12 | MUNDER CAPITAL MANAGEMENT | NORTHWESTERN CORP | 3,445,932 | 9.6% | EDGAR |
| SC 13G/A | 2009-02-10 | RS INVESTMENT MANAGEMENT CO LLC | NORTHWESTERN CORP | - | 0.3% | EDGAR |
| SC 13G/A | 2009-01-30 | KING STREET CAPITAL MANAGEMENT, L.P. | NORTHWESTERN CORP | 0 | 0.0% | EDGAR |
| SC 13G/A | 2008-02-14 | ZIMMER LUCAS CAPITAL LLC | NORTHWESTERN CORP | 2,415,757 | 6.5% | EDGAR |
| SC 13G/A | 2008-02-14 | ANGELO GORDON & CO LP/NY | NORTHWESTERN CORP | 1,200,000 | 3.2% | EDGAR |
| SC 13G/A | 2008-02-13 | LEHMAN BROTHERS HOLDINGS INC | NORTHWESTERN CORP | 126,122 | 0.3% | EDGAR |
| SC 13G | 2008-02-08 | RS INVESTMENT MANAGEMENT CO LLC | NORTHWESTERN CORP | - | 7.1% | EDGAR |
| SC 13G/A | 2008-02-06 | RAMIUS CAPITAL GROUP LLC | NORTHWESTERN CORP | 2,287,400 | 6.2% | EDGAR |