Equity Investments (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended				12 Months Ended			1 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Less than	Dec. 31, 2012 GOV	Dec. 31, 2011 GOV	Dec. 31, 2010 GOV	Oct. 31, 2012 GOV Subsequent Event	Oct. 31, 2012 GOV Less than	Dec. 31, 2012 AIC	Dec. 31, 2011 AIC	Dec. 31, 2010 AIC	Dec. 31, 2012 AIC SIR
Equity Investments
Subsidiary's attribution to equity method investment													100.00%
Equity method investments, ownership percentage				50.00%	18.20%	21.10%			20.00%	25.00%	14.30%		12.50%
Equity investments	$ 184,711	$ 177,477			$ 173,452	$ 172,186				$ 11,259	$ 5,291
Equity in Earnings (Loss)	11,420	11,377	8,464		10,836	11,238	8,465			584	139	(1)
Equity investments, common shares owned					9,950,000
Equity investments, market value based on quoted market prices					238,502
Market value of common shares exceeded carrying value					13,824
Common shares issued								7,500,000
Common stock price per share (in dollars per share)								$ 23.25
Net proceeds received on issuance of common shares								166,700
Gain on issuance of shares by an equity investee	7,246	11,177	34,808					7,246
Ownership percentage after the transaction (as a percent)								18.20%
Amortizing the difference between carrying value and share of underlying equity					30 years
Cash distributions					16,816	16,617	16,119
Amount invested in equity investee										10,544
Combined carrying value of equity method investments										$ 11,259