Summarized Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 1,136,791	$ 1,049,111	$ 993,704	$ 1,203,543	$ 1,074,442	$ 1,044,300	$ 966,429	$ 1,117,457	$ 4,383,149	$ 4,202,628	$ 3,969,293
Operating income	169,423	261,073	163,437	208,437	143,044	289,741	183,675	212,299	802,370	828,759	849,219
Net income	$ 92,834	$ 154,159	$ 89,792	$ 118,403	$ 73,213	$ 166,266	$ 97,299	$ 116,605	$ 455,188	$ 453,383	$ 458,076