Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Qualified Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 8,575	$ 6,466	$ 17,149	$ 12,932	$ 30,081	$ 27,209
Interest cost	11,389	12,251	22,777	24,503	47,280	46,590
Expected return on plan assets	(16,324)	(15,061)	(32,648)	(30,122)	(62,770)	(59,500)
Amortization of net actuarial loss	9,006	5,589	18,013	11,178	29,191	27,236
Net periodic benefit cost	12,646	9,245	25,291	18,491	43,782	41,535
SERP
Defined Benefit Plan Disclosure [Line Items]
Service cost	97	67	195	133	328	256
Interest cost	402	440	803	880	1,683	1,709
Amortization of net actuarial loss	451	255	902	510	1,412	1,261
Net periodic benefit cost	950	762	1,900	1,523	3,423	3,226
PBOP
Defined Benefit Plan Disclosure [Line Items]
Service cost	395	319	791	638	1,429	1,374
Interest cost	645	762	1,290	1,524	2,812	2,898
Expected return on plan assets	(852)	(789)	(1,704)	(1,578)	(3,282)	(3,436)
Amortization of prior service costs	289	318	578	635	1,214	1,302
Net periodic benefit cost	$ 477	$ 610	$ 955	$ 1,219	$ 2,173	$ 2,138