Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Net Income	$ 80,433	$ 79,570	$ 117,345	$ 131,036
Other comprehensive income (loss), net:
Net change related to employee benefit plans, net of tax expense	1,390	1,228	2,858	2,155
Net change in derivative instruments, net of tax benefit	1,261	(7,080)	(5,836)	(15,308)
Net change in available-for-sale investments, net of tax expense	32	151	118	683
Total other comprehensive income (loss)	2,683	(5,701)	(2,860)	(12,470)
Total Comprehensive Income	$ 83,116	$ 73,869	$ 114,485	$ 118,566