Quarterly Results of Operations (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 192,803	$ 184,511	$ 174,888	$ 167,576	$ 169,733	$ 181,967	$ 178,033	$ 180,965	$ 719,778	$ 710,698	$ 773,960
Net income	$ 131,824	$ 61,159	$ 52,818	$ 44,244	$ 200,291	$ 55,734	$ 51,057	$ 59,063	$ 290,045	$ 366,145	$ 245,924
Net income available to common shareholders - basic (USD per share/unit)	$ 0.87	$ 0.41	$ 0.35	$ 0.29	$ 1.33	$ 0.37	$ 0.34	$ 0.39
Income per common share - diluted (USD per share/unit)	$ 0.87	$ 0.40	$ 0.35	$ 0.29	$ 1.33	$ 0.37	$ 0.34	$ 0.39