Components of Net Periodic Benefit Cost - Schedule of Components of Net Periodic Benefit Costs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service cost			$ (177)
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5,764	5,080	11,528	10,159	21,688	19,022
Interest cost	9,402	9,566	18,803	19,133	37,936	37,771
Expected return on plan assets	(12,460)	(11,445)	(24,920)	(22,890)	(47,810)	(42,947)
Amortization of net actuarial loss	8,065	5,971	16,131	11,942	28,072	19,116
Net periodic benefit cost	10,771	9,172	21,542	18,344	39,886	32,962
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	93	68	187	137	324	245
Interest cost	384	407	767	814	1,582	1,697
Amortization of net actuarial loss	243	171	486	342	827	658
Net periodic benefit cost	720	646	1,440	1,293	2,733	2,600
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	305	244	610	489	1,098	918
Interest cost	621	637	1,241	1,273	2,515	2,588
Expected return on plan assets	(706)	(601)	(1,412)	(1,202)	(2,614)	(2,391)
Amortization of prior service cost	88		177		177
Amortization of transition obligation		216		433	434	867
Amortization of net actuarial loss	237	258	473	516	988	811
Net periodic benefit cost	$ 545	$ 754	$ 1,089	$ 1,509	$ 2,598	$ 2,793