Components of Net Periodic Benefit Cost - Schedule of Net Periodic Benefit Costs (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7,139	$ 5,848	$ 24,683	$ 22,972
Interest cost	11,043	11,520	45,606	46,041
Expected return on plan assets	(14,689)	(13,799)	(56,086)	(56,217)
Amortization of net actuarial loss	8,029	6,001	26,032	24,950
Net periodic benefit cost	11,522	9,570	40,235	37,746
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	61	78	292	327
Interest cost	415	471	1,827	1,865
Amortization of net actuarial loss	375	360	1,456	1,397
Net periodic benefit cost	851	909	3,575	3,589
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	368	367	1,469	1,492
Interest cost	687	808	3,111	3,192
Expected return on plan assets	(930)	(840)	(3,448)	(3,201)
Amortization of prior service costs	334	334	1,335	1,335
Amortization of net actuarial loss				313
Net periodic benefit cost	$ 459	$ 669	$ 2,467	$ 3,131