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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Information [Abstract]
Total revenues	$ 76,600	$ 92,000	$ 63,000	$ 43,600	$ 76,300	$ 44,600	$ 86,700	$ 79,600	$ 275,234	$ 287,152	$ 285,079
Total expenses	52,600	61,400	50,500	43,200	55,200	41,600	61,300	57,900	207,764	215,946	214,604
Income before taxes	24,000	30,600	12,500	400	21,100	3,000	25,400	21,700	67,470	71,206	70,475
Provision for taxes	8,700	10,800	4,500	100	5,300	1,200	9,900	8,100	24,082	24,524	28,383
Net income allocated to common stockholders	$ 15,300	$ 19,800	$ 8,000	$ 300	$ 15,800	$ 1,800	$ 15,500	$ 13,600	$ 43,388	$ 46,682	$ 42,092
Basic (usd per share)	$ 0.51	$ 0.66	$ 0.27	$ 0.01	$ 0.53	$ 0.06	$ 0.52	$ 0.45	$ 1.43	$ 1.55	$ 1.38
Diluted earnings per share	$ 0.51	$ 0.66	$ 0.27	$ 0.01	$ 0.53	$ 0.06	$ 0.52	$ 0.45	$ 1.43	$ 1.55	$ 1.38
Dividends declared per share (usd per share)	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 1.80	$ 1.80	$ 1.80