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 Financial Information Disclosure [Abstract]
Operating revenue	$ 179,057	$ 190,857	$ 186,654	$ 190,140	$ 199,382	$ 198,972	$ 203,518	$ 206,901	$ 746,708	$ 808,773	$ 792,631
Net income	$ 200,291	$ 55,734	$ 51,057	$ 59,063	$ 83,841	$ 93,695	$ 36,526	$ 31,862	$ 366,145	$ 245,924	$ 224,471
Net income available to common shareholders - basic (USD per share/unit)	$ 1.33	$ 0.37	$ 0.34	$ 0.39	$ 0.56	$ 0.61	$ 0.24	$ 0.21
Income per common share - diluted (USD per share/unit)	$ 1.33	$ 0.37	$ 0.34	$ 0.39	$ 0.55	$ 0.61	$ 0.24	$ 0.21