Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	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, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 1,420,460	$ 1,342,764	$ 1,304,605	$ 1,302,154	$ 1,335,755	$ 1,299,286	$ 1,258,971	$ 1,239,655	$ 1,279,785	$ 1,191,665	$ 1,031,769	$ 819,864	$ 5,369,983	$ 5,133,667	$ 4,323,083
Net income	198,273	104,410	92,453	194,006	118,222	76,824	41,141	87,501	98,939	66,967	28,676	89,263	589,142	323,688	283,845
Net income to shareholders	197,892	102,519	91,369	190,992	117,595	75,803	40,068	87,716	98,764	65,599	27,756	88,902	582,772	321,182	281,021
Comprehensive income (loss) to shareholders	$ 134,984	$ (51,143)	$ (132,925)	$ 281,807	$ 418,571	$ 36,502	$ 250,588	$ 230,273	$ 206,450	$ 144,409	$ (149,054)	$ 257,684	$ 232,723	$ 935,934	$ 459,489
Basic net income per share (dollars per share)	$ 14.23	$ 7.43	$ 6.76	$ 13.57	$ 8.10	$ 5.33	$ 2.67	$ 6.28	$ 6.98	$ 4.69	$ 2.24	$ 9.53	$ 41.99	$ 22.38	$ 22.57
Diluted net income per share (dollars per share)	14.14	7.39	6.72	13.49	8.05	5.30	2.66	6.25	6.95	4.67	2.24	9.50	$ 41.74	$ 22.27	$ 22.48
Common stock price, high (dollars per share)	937.91	898.08	821.00	783.50	707.36	666.00	655.75	596.87	582.59	549.09	546.94	510.05
Common stock price, low (dollars per share)	$ 791.97	$ 775.00	$ 736.96	$ 660.05	$ 632.65	$ 623.90	$ 593.76	$ 527.17	$ 511.06	$ 506.64	$ 501.76	$ 434.98