Components of Net Periodic Benefit Cost - Schedule of Components of Net Periodic Benefit Costs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,280	$ 5,340	$ 22,300	$ 22,632
Interest cost	11,058	10,861	43,637	39,067
Expected return on plan assets	(14,452)	(13,336)	(54,458)	(50,716)
Amortization of net actuarial loss	8,185	5,718	25,340	29,913
Net periodic benefit cost	11,071	8,583	36,819	40,896
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	80	73	299	352
Interest cost	423	436	1,732	1,588
Amortization of net actuarial loss	324	196	911	924
Net periodic benefit cost	827	705	2,942	2,864
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	411	276	1,236	1,191
Interest cost	749	707	2,871	2,569
Expected return on plan assets	(866)	(816)	(3,314)	(2,934)
Amortization of prior service costs	333	89	599	355
Amortization of net actuarial loss	87		87	709
Net periodic benefit cost	$ 714	$ 256	$ 1,479	$ 1,890