Income Taxes - Income Tax Provisions Included in the Consolidated Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Current expense:
Federal									$ 32,378	$ 37,623	$ 23,341
State									4,048	914	518
Deferred expense:
Federal and State									2,990	337	10,907
Total income taxes	$ 9,252	$ 10,433	$ 9,576	$ 10,155	$ 9,983	$ 9,099	$ 9,905	$ 9,887	$ 39,416	$ 38,874	$ 34,766