Components of Net Periodic Benefit Cost - Schedule of Components of Net Periodic Benefit Costs (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,281	$ 5,340	$ 18,842	$ 16,020	$ 24,182	$ 21,784
Interest cost	11,057	10,860	33,172	32,581	44,031	41,983
Expected return on plan assets	(14,452)	(13,336)	(43,356)	(40,007)	(56,691)	(52,467)
Amortization of net actuarial loss	8,186	5,718	24,557	17,154	30,276	25,219
Net periodic benefit cost	11,072	8,582	33,215	25,748	41,798	36,519
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	80	73	240	219	313	312
Interest cost	424	436	1,271	1,308	1,708	1,692
Amortization of net actuarial loss	323	196	970	588	1,165	830
Net periodic benefit cost	827	705	2,481	2,115	3,186	2,834
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	410	275	1,231	826	1,506	1,131
Interest cost	750	707	2,249	2,122	2,956	2,743
Expected return on plan assets	(866)	(816)	(2,598)	(2,448)	(3,414)	(3,154)
Amortization of prior service costs	334	89	1,001	266	1,090	355
Amortization of net actuarial loss	86	0	259	0	259	236
Net periodic benefit cost	$ 714	$ 255	$ 2,142	$ 766	$ 2,397	$ 1,311