Components of Net Periodic Benefit Cost - Schedule of Components of Net Periodic Benefit Costs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service cost			$ (266)
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5,764	5,080	17,292	15,239	22,372	19,671
Interest cost	9,402	9,567	28,205	28,700	37,771	38,019
Expected return on plan assets	(12,460)	(11,445)	(37,380)	(34,335)	(48,825)	(44,364)
Amortization of net actuarial loss	8,065	5,970	24,196	17,912	30,167	21,499
Net periodic benefit cost	10,771	9,172	32,313	27,516	41,485	34,825
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	93	69	280	206	348	260
Interest cost	384	407	1,151	1,221	1,559	1,663
Amortization of net actuarial loss	243	171	729	513	899	671
Net periodic benefit cost	720	647	2,160	1,940	2,806	2,594
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	305	244	915	733	1,159	947
Interest cost	620	637	1,861	1,910	2,498	2,567
Expected return on plan assets	(706)	(601)	(2,118)	(1,803)	(2,719)	(2,397)
Amortization of prior service cost	89		266		266
Amortization of transition obligation		217		650	217	867
Amortization of net actuarial loss	236	257	709	773	967	921
Net periodic benefit cost	$ 544	$ 754	$ 1,633	$ 2,263	$ 2,388	$ 2,905