Quarterly Results of Operations (Unaudited) (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
Quarterly Results of Operations
Total revenue	$ 206,958	$ 210,085	$ 202,950	$ 199,829	$ 201,042	$ 201,270	$ 197,602	$ 198,039	$ 819,822	$ 797,953	$ 778,797
Net income (loss)	$ 11,953	$ 18,597	$ 21,365	$ 12,670	$ 10,290	$ 11,981	$ (1,053)	$ (35,071)	$ 64,585	$ (13,853)	$ 59,354
Earnings (loss) per weighted average common share:
Basic (in dollars per share)	$ 0.79	$ 1.24	$ 1.42	$ 0.84	$ 0.69	$ 0.80	$ (0.07)	$ (2.37)	$ 4.29	$ (0.93)	$ 3.93
Diluted (in dollars per share)	0.79	1.23	1.41	0.84	0.69	0.80	(0.07)	(2.37)	4.27	(0.93)	3.91
Cash dividends paid per common share (in dollars per share)	$ 0.70	$ 0.70	$ 0.70	$ 0.70	$ 0.70	$ 0.70	$ 0.70	$ 0.70	$ 2.80	$ 2.80	$ 2.60