Debt - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013
Debt Disclosure [Abstract]
Senior Notes principal amount	$ 600,000
Senior Notes effective interest rate	3.97%
Senior Notes maturity	Sep. 15, 2022
Senior Notes interest rate	3.95%
Commencement date of Senior Notes payment	Mar. 15, 2013
Frequency of Senior Notes payment	Semi-annually in arrears on March 15 and September 15
Repurchase agreement maximum loan borrowing capacity	25,000
Repurchase Agreement outstanding amount	0
Repurchase Agreement borrowing base limitations	$ 0
Expiration date of Repurchase Agreement	Jul. 30, 2014