Employee Benefit Plans - Changes in Projected Benefit Obligations and Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension [Member]
Change in Projected Benefit Obligation
Projected benefit obligation at beginning of year	$ 409,356	$ 173,077
Service cost	5,977	4,099	$ 2,790
Interest cost	15,228	11,379	6,900
Actuarial loss (gain)	(7,073)	45,171
Plan amendments	0	0
Employee contributions	0	0
Benefits paid	(18,455)	(13,468)
Assumption of balance from parent at spin	0	189,098
Projected benefit obligation at end of year	405,033	409,356	173,077
Defined Benefit Plan, Information about Plan Assets [Abstract]
Fair value of plan assets at beginning of year	291,087	170,218
Actual return on plan assets	(6,627)	13,359
Employer contributions	2,312	1,056
Other expense	(2,162)	(1,175)
Assumption of balance from parent at spin	0	121,097
Fair value of plan assets at end of year	266,155	291,087	170,218
Funded Status at End of Year:
Net accrued benefit cost	(138,878)	(118,269)
Postretirement [Member]
Change in Projected Benefit Obligation
Projected benefit obligation at beginning of year	26,568	17,178
Service cost	1,006	798	941
Interest cost	919	916	741
Actuarial loss (gain)	(2,049)	4,417
Plan amendments	1,321	0
Employee contributions	361	0
Benefits paid	(1,167)	(1,309)
Assumption of balance from parent at spin	0	4,568
Projected benefit obligation at end of year	26,959	26,568	17,178
Defined Benefit Plan, Information about Plan Assets [Abstract]
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	806	1,309
Other expense	0	0
Assumption of balance from parent at spin	0	0
Fair value of plan assets at end of year	0	0	$ 0
Funded Status at End of Year:
Net accrued benefit cost	$ (26,959)	$ (26,568)