Employee Benefit Plans - Postretirement Healthcare Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure
Amortization of gain, weighted average remaining service period (in years)		4 years
Stockholders’ equity	$ 1,427,616	$ 982,593	$ 950,493	$ 908,924
Postretirement Healthcare Plan
Defined Benefit Plan Disclosure
Minimum age requirement	55 years
Requisite service period	15 years
Accumulated other comprehensive income (loss), before tax, related to the Plan	$ 2,200	2,800
Stockholders’ equity	1,300
Reduction in accumulated post-retirement benefit obligation due to curtailment		2,800
Reduction in net periodic benefit cost due to curtailment		300
Unfunded benefit obligation	700	700
Net periodic benefit cost	$ 400	$ 200	$ 400
Assumptions used calculating net periodic benefit cost, discount rate	2.40%	2.40%
Assumptions used calculating net periodic benefit cost, rate of compensation increase	6.00%	6.00%
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2018	$ 130
2019	130
2020	100
2021	70
2022	50
2023 through 2027	100
Net actuarial gains	2,100
Unamortized negative prior service costs	100
Estimated amount amortized from accumulated other comprehensive loss into net periodic benefit costs	$ 800