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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,708	$ 6,281	$ 17,125	$ 18,842	$ 23,406	$ 24,182
Interest cost	11,507	11,057	34,520	33,172	45,577	44,031
Expected return on plan assets	(14,140)	(14,452)	(42,419)	(43,356)	(56,871)	(56,691)
Amortization of net actuarial loss	6,317	8,186	18,950	24,557	27,136	30,276
Net periodic benefit cost	9,392	11,072	28,176	33,215	39,248	41,798
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	83	80	248	240	328	313
Interest cost	464	424	1,394	1,271	1,818	1,708
Amortization of net actuarial loss	346	323	1,038	970	1,361	1,165
Net periodic benefit cost	893	827	2,680	2,481	3,507	3,186
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	375	410	1,124	1,231	1,534	1,506
Interest cost	795	750	2,386	2,249	3,136	2,956
Expected return on plan assets	(787)	(866)	(2,362)	(2,598)	(3,228)	(3,414)
Amortization of prior service costs	333	334	1,001	1,001	1,335	1,090
Amortization of net actuarial loss	104	86	312	259	398	259
Net periodic benefit cost	$ 820	$ 714	$ 2,461	$ 2,142	$ 3,175	$ 2,397