Income Taxes - Provision for income taxes breakup - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Provisional income tax expense	$ 26,600
Current:
Federal	46,153	$ 37,187	$ 41,527
State	3,018	3,325	2,327
Total current tax expense	49,171	40,512	43,854
Deferred:
Federal	31,971	11,684	6,344
State	109	564	704
Total deferred tax expense (benefit)	32,080	12,248	7,048
Provision for income taxes	$ 81,251	52,760	50,902
Federal statutory income tax rate (as a percent)	35.00%
Parent company
Deferred:
Provision for income taxes	$ (5,053)	$ (4,270)	$ (2,975)