Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Qualified Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 8,574	$ 6,466	$ 27,972	$ 27,882
Interest cost	11,388	12,252	48,142	45,383
Expected return on plan assets	(16,324)	(15,061)	(61,507)	(59,127)
Amortization of net actuarial loss	9,007	5,589	25,774	29,675
Net periodic benefit cost	12,645	9,246	40,381	43,813
SERP
Defined Benefit Plan Disclosure [Line Items]
Service cost	98	66	298	250
Interest cost	401	440	1,721	1,683
Amortization of net actuarial loss	451	255	1,216	1,382
Net periodic benefit cost	950	761	3,235	3,315
PBOP
Defined Benefit Plan Disclosure [Line Items]
Service cost	396	319	1,353	1,424
Interest cost	645	762	2,929	2,823
Expected return on plan assets	(852)	(789)	(3,219)	(3,577)
Amortization of prior service costs	289	317	1,243	1,318
Net periodic benefit cost	$ 478	$ 609	$ 2,306	$ 1,988