Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Qualified Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,466	$ 7,139	$ 27,882	$ 24,683
Interest cost	12,252	11,043	45,383	45,606
Expected return on plan assets	(15,061)	(14,689)	(59,127)	(56,086)
Amortization of net actuarial loss	5,589	8,029	29,675	26,032
Net periodic benefit cost	9,246	11,522	43,813	40,235
SERP
Defined Benefit Plan Disclosure [Line Items]
Service cost	66	61	250	292
Interest cost	440	415	1,683	1,827
Amortization of net actuarial loss	255	375	1,382	1,456
Net periodic benefit cost	761	851	3,315	3,575
PBOP
Defined Benefit Plan Disclosure [Line Items]
Service cost	319	368	1,424	1,469
Interest cost	762	687	2,823	3,111
Expected return on plan assets	(789)	(930)	(3,577)	(3,448)
Amortization of prior service costs	317	334	1,318	1,335
Net periodic benefit cost	$ 609	$ 459	$ 1,988	$ 2,467