Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Qualified Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 8,576	$ 6,466	$ 25,725	$ 19,398	$ 32,191	$ 26,536
Interest cost	11,388	12,252	34,165	36,755	46,416	47,799
Expected return on plan assets	(16,324)	(15,061)	(48,972)	(45,183)	(64,033)	(59,872)
Amortization of net actuarial loss	9,006	5,589	27,019	16,767	32,608	24,796
Net periodic benefit cost	12,646	9,246	37,937	27,737	47,182	39,259
SERP
Defined Benefit Plan Disclosure [Line Items]
Service cost	97	66	292	199	359	261
Interest cost	401	440	1,204	1,320	1,644	1,734
Amortization of net actuarial loss	451	255	1,353	765	1,608	1,141
Net periodic benefit cost	949	761	2,849	2,284	3,611	3,136
PBOP
Defined Benefit Plan Disclosure [Line Items]
Service cost	395	319	1,186	957	1,505	1,325
Interest cost	646	761	1,936	2,285	2,697	2,972
Expected return on plan assets	(852)	(789)	(2,556)	(2,367)	(3,345)	(3,296)
Amortization of prior service costs	289	318	867	953	1,185	1,286
Net periodic benefit cost	$ 478	$ 609	$ 1,433	$ 1,828	$ 2,042	$ 2,287