Income Taxes Components of current and deferred income tax expense (benefit) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current provision (benefit)
Federal									$ 81,101	$ 9,464	$ 8,760
State and other									(11,801)	(13,104)	1,474
Current Income Tax Expense (Benefit)									69,300	(3,640)	10,234
Deferred provision (benefit)
Federal									444,695	312,288	277,895
State and other									(22,388)	22,499	33,804
Deferred Income Tax Expense (Benefit)									422,307	334,787	311,699
Income tax expense (benefit)	$ 331,352	$ 90,710	$ 21,798	$ 47,747	$ 140,549	$ 83,865	$ 71,820	$ 34,913	$ 491,607	$ 331,147	$ 321,933