Supplemental Financial Information Quarterly Results Supplemental Financial Information - Schedule of Unaudited Quarterly Results (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Selected Quarterly Financial Information [Abstract]
Total revenues	$ 75,700	$ 50,600	$ 73,300	$ 61,900	$ 76,600	$ 92,000	$ 63,000	$ 43,600	$ 261,560	$ 275,234	$ 287,152
Total expenses	61,500	49,500	57,600	49,700	52,600	61,400	50,500	43,200	218,265	207,764	215,946
Income before taxes	14,200	1,100	15,700	12,200	24,000	30,600	12,500	400	43,295	67,470	71,206
Provision for taxes	6,300	500	6,400	4,600	8,700	10,800	4,500	100	17,697	24,082	24,524
Net income allocated to common stockholders	$ 7,900	$ 600	$ 9,300	$ 7,600	$ 15,300	$ 19,800	$ 8,000	$ 300	$ 25,598	$ 43,388	$ 46,682
Basic (usd per share)	$ 0.25	$ 0.02	$ 0.30	$ 0.25	$ 0.51	$ 0.66	$ 0.27	$ 0.01	$ 0.82	$ 1.43	$ 1.55
Diluted (usd per share)	0.25	0.02	0.30	0.25	0.51	0.66	0.27	0.01	0.82	1.43	1.55
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