Quarterly Financial Data Table of Quarterly Results (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, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating revenues	$ 514,853	$ 546,299	$ 542,219	$ 397,030	$ 415,627	$ 432,543	$ 414,563	$ 376,279
Income from operations	94,631	117,559	99,451	72,620	113,471	123,116	110,109	74,875	$ 384,261	$ 421,571	$ 380,188
NET INCOME	$ 84,934	$ 63,362	$ 52,837	$ 23,904	$ 87,373	$ 72,650	$ 60,728	$ 40,004	$ 225,037	$ 260,755	$ 271,296
Diluted earnings per common share (US$ per share)	$ 1.47	$ 1.10	$ 0.92	$ 0.42	$ 1.53	$ 1.27	$ 1.07	$ 0.70	$ 3.89	$ 4.58	$ 4.79