Supplemental Financial Information Quarterly Results Supplemental Financial Information - Schedule of Unaudited Quarterly Results (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Selected Quarterly Financial Information [Abstract]
Total revenues	$ 92,300	$ 62,700	$ 47,300	$ 82,700	$ 94,500	$ 60,400	$ 90,700	$ 48,400	$ 285,079	$ 293,993	$ 278,329
Total expenses	64,200	48,800	43,800	57,900	69,400	47,400	57,600	51,000	214,604	225,323	219,378
Income before taxes	28,100	13,900	3,500	24,800	25,100	13,000	33,100	(2,600)	70,475	68,670	58,951
Provision (benefit) for taxes	13,000	5,300	1,300	8,700	9,000	4,400	11,600	(1,000)	28,383	24,086	19,530
Net income allocated to common stockholders	$ 15,100	$ 8,600	$ 2,200	$ 16,100	$ 16,100	$ 8,600	$ 21,500	$ (1,600)	$ 42,092	$ 44,578	$ 34,526
Basic	$ 0.50	$ 0.28	$ 0.07	$ 0.53	$ 0.53	$ 0.28	$ 0.69	$ (0.05)	$ 1.38	$ 1.44	$ 1.12
Diluted earnings per share	$ 0.50	$ 0.28	$ 0.07	$ 0.53	$ 0.53	$ 0.28	$ 0.69	$ (0.05)	$ 1.38	$ 1.44	$ 1.12
Dividends declared per share	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ (1.80)	$ (1.80)	$ (1.80)