Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016	[1]
Quarterly Financial Data (Unaudited)
Operating revenues	$ 803,489	$ 829,275	$ 805,515	$ 783,400	$ 771,241	$ 812,673	$ 791,512	$ 747,166	$ 3,221,679	$ 3,122,592	[1]	$ 3,063,702
Operating income	121,502	137,925	126,678	88,175	92,939	112,369	106,596	76,295	474,280	388,199	[1]	(172,684)
Net income	$ 67,105	$ 83,046	$ 75,859	$ 54,362	$ 289,928	$ 53,716	$ 50,477	$ 34,786	$ 280,372	$ 428,907	[1]	$ (161,586)
Net income per common share:
Basic (in dollars per share)	$ 1.30	$ 1.60	$ 1.46	$ 1.05	$ 5.60	$ 1.04	$ 0.98	$ 0.67	$ 5.40	$ 8.28	[1]	$ (3.14)
Diluted (in dollars per share)	$ 1.28	$ 1.57	$ 1.43	$ 1.03	$ 5.46	$ 1.01	$ 0.95	$ 0.65	$ 5.30	$ 8.08	[1]	$ (3.14)
Weighted average common shares:
Basic (in shares)	51,650	52,039	52,046	51,921	51,811	51,833	51,751	51,820	51,914	51,804	[1]	51,505
Diluted (in shares)	52,556	52,981	52,913	53,033	53,140	53,080	52,977	53,202	52,871	53,100	[1]	51,505
Revaluation of deferred tax liability										$ 246,800

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements of this report for additional information.