Basis of Presentation - Consolidated Statements of Changes In Equity (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Accumulated other comprehensive (loss) income:
Beginning Balance	$ 56,182		$ 85,264	$ 60,329
Other comprehensive income				1,733
Ending balance	96,295	62,062	96,295	62,062
Retained earnings:
Beginning Balance			1,744,684	1,514,866
Net income	90,512	109,459	257,238	273,810
Cash dividends declared				(53,567)
Ending Balance	1,363,970	1,735,109	1,363,970	1,735,109
As Previously Reported
Accumulated other comprehensive (loss) income:
Beginning Balance				(222,922)
Other comprehensive income				30,507
Ending balance		(192,415)		(192,415)
Retained earnings:
Beginning Balance				1,798,117
Net income		99,261		245,036
Cash dividends declared				(53,567)
Ending Balance		1,989,586		1,989,586
Effect of Accounting Change
Accumulated other comprehensive (loss) income:
Beginning Balance				283,251
Other comprehensive income				(28,774)
Ending balance		254,477		254,477
Retained earnings:
Beginning Balance				(283,251)
Net income		10,198		28,774
Cash dividends declared				0
Ending Balance		$ (254,477)		$ (254,477)