Components of Net Periodic Benefit Cost - Schedule of Components of Net Periodic Benefit Costs (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,281	$ 5,340	$ 12,561	$ 10,680	$ 23,241	$ 22,208
Interest cost	11,057	10,860	22,115	21,721	43,834	40,525
Expected return on plan assets	(14,452)	(13,335)	(28,904)	(26,671)	(55,575)	(51,591)
Amortization of net actuarial loss	8,186	5,718	16,371	11,436	27,808	27,566
Net periodic benefit cost	11,072	8,583	22,143	17,166	39,308	38,708
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	80	73	160	146	306	332
Interest cost	424	436	847	872	1,720	1,640
Amortization of net actuarial loss	323	196	647	392	1,038	877
Net periodic benefit cost	827	705	1,654	1,410	3,064	2,849
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	410	275	821	551	1,371	1,161
Interest cost	750	708	1,499	1,415	2,913	2,656
Expected return on plan assets	(866)	(816)	(1,732)	(1,632)	(3,364)	(3,044)
Amortization of prior service costs	334	88	667	177	845	355
Amortization of net actuarial loss	86	0	173	0	173	472
Net periodic benefit cost	$ 714	$ 255	$ 1,428	$ 511	$ 1,938	$ 1,600