Supplemental Financial Information Quarterly Results (unaudited) - Schedule of Unaudited Quarterly Results (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Selected Quarterly Financial Information [Abstract]
Total revenues	$ 101,600	$ 76,600	$ 90,500	$ 66,900	$ 75,700	$ 50,600	$ 73,300	$ 61,900	$ 335,519	$ 261,560	$ 275,234
Total expenses	69,200	57,400	60,600	60,400	61,500	49,500	57,600	49,700	247,638	218,265	207,764
Income before taxes	32,400	19,200	29,900	6,500	14,200	1,100	15,700	12,200	87,881	43,295	67,470
Provision for taxes	8,700	6,100	10,200	2,100	6,300	500	6,400	4,600	27,119	17,697	24,082
Net income allocated to common stockholders	$ 23,700	$ 13,100	$ 19,700	$ 4,400	$ 7,900	$ 600	$ 9,300	$ 7,600	$ 60,762	$ 25,598	$ 43,388
Basic (usd per share)	$ 0.74	$ 0.41	$ 0.62	$ 0.14	$ 0.25	$ 0.02	$ 0.30	$ 0.25	$ 1.90	$ 0.82	$ 1.43
Diluted (usd per share)	0.74	0.41	0.62	0.14	0.25	0.02	0.30	0.25	1.89	0.82	1.43
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