Equity Investments (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Equity Investments
Equity investments	$ 176,255		$ 177,477
Equity in Earnings (Loss)	2,958	2,712
GOV
Equity Investments
Equity method investments, ownership percentage	21.10%		21.10%
Equity investments	170,920		172,186
Equity in Earnings (Loss)	2,913	2,675
Equity investments, common shares owned	9,950,000
Equity investments, market value based on quoted market prices	239,895
Equity investments, quoted market price of common share (in dollars per share)	$ 24.11		$ 22.55
Market value of common shares exceeded carrying value	13,824
Amortizing the difference between carrying value and share of underlying equity (in years)	30
Cash distributions	4,179	4,080
AIC
Equity Investments
Equity method investments, ownership percentage	14.30%		14.30%
Equity investments	5,335		5,291
Equity in Earnings (Loss)	45	37
Amount invested in equity investee	$ 5,209
AIC | Less than
Equity Investments
Equity method investments, ownership percentage			20.00%