Pension Plan and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension Plan
Pension Plan and Other Postretirement Benefits
Employer cash contributions			$ 12,500	$ 42,300
Estimated cash contributions in the current fiscal year	$ 18,800		18,800
Components of the pension plans and other postretirement benefits
Service cost	6,910	$ 6,966	20,039	21,770
Interest cost	6,941	6,007	20,225	17,996
Expected return on plan assets	(7,581)	(7,052)	(22,714)	(20,777)
Amortization of prior service cost	1,262	1,263	3,786	3,789
Recognized net actuarial loss	1,821	2,791	4,445	8,386
Net periodic benefit cost	9,353	9,975	25,781	31,164
Other Benefits
Pension Plan and Other Postretirement Benefits
Employer cash contributions			5,600	8,000
Estimated cash contributions in the current fiscal year	7,900		7,900
Components of the pension plans and other postretirement benefits
Service cost	2,082	1,966	5,606	7,066
Interest cost	1,407	1,183	4,081	4,152
Expected return on plan assets	(1,475)	(1,397)	(4,346)	(4,229)
Amortization of prior service cost	49	11	148	32
Recognized net actuarial loss	214	242	421	1,789
Net periodic benefit cost	$ 2,277	$ 2,005	$ 5,910	$ 8,810