- Employee Benefits - Additional Information (Details)			12 Months Ended
	Sep. 07, 2018 USD ($)	Dec. 19, 2017 USD ($) retiree	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Number of consecutive years of employee's compensation			5 years
Number of total years of employee's compensation			10 years
Service cost			$ 0	$ 0	$ 463,000
Expected long-term rate of return on plan assets			7.50%	7.50%	7.50%
Employer contributions	$ 20,400,000		$ 20,420,000	$ 0
Return to provision discrete tax benefit			2,900,000
Net actuarial loss included in accumulated other comprehensive losses expected to be recognized			1,600,000
Future prior service credit to be recognized due to freeze of accrual benefits			0
Accumulated benefit obligation			95,200,000	106,700,000
Number of retirees covered by annuity contract | retiree		124
Number of retirees covered by annuity contract receiving death benefits | retiree		30
Defined benefit obligations, death benefits		$ 2,000
Total premium paid out of plan's assets		$ 1,500,000	7,975,000	8,244,000
Projected benefit obligation			95,200,000	106,664,000	$ 105,834,000
Net periodic benefit cost			(250,000)	(347,000)	296,000
Transfers of plan assets between levels 1 and 2 on recurring basis			2,200,000	0
Purchases, transfers or sales of level 3 plan assets			0
Supplemental Executive Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation			4,400,000	5,300,000
Net periodic benefit cost			500,000	500,000	500,000
Amounts before tax reflected in accumulated other comprehensive income (loss)			$ 1,900,000	2,700,000	2,500,000
Thrift Plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to the Thrift Plan			3.50%
Compensation expense			$ 1,700,000	$ 1,800,000	$ 1,700,000
Total equity securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Targeted asset allocation percentage			5.00%	50.00%
Total equity securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Targeted asset allocation percentage			15.00%	70.00%
Fixed income | Minimum
Defined Benefit Plan Disclosure [Line Items]
Targeted asset allocation percentage			85.00%	30.00%
Fixed income | Maximum
Defined Benefit Plan Disclosure [Line Items]
Targeted asset allocation percentage			95.00%	50.00%