Debt - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 31, 2012	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Prior Credit Agreement
Line of Credit Facility [Line Items]
Maximum borrowing capacity			150,000		150,000
Number of credit facilities under the Prior Credit Agreement			2		2
Percentage of daily amount of commitment paid as facility fee under the Prior Credit Agreement			0.25%		0.25%
Prior Credit Agreement | Revolving Credit Facility, One
Line of Credit Facility [Line Items]
Maximum borrowing capacity			50,000		50,000
Prior Credit Agreement | Revolving Credit Facility, Two
Line of Credit Facility [Line Items]
Maximum borrowing capacity			100,000		100,000
New Credit Agreement
Line of Credit Facility [Line Items]
Maximum borrowing capacity	325,000	325,000		325,000
Additional revolving commitments or term loan commitments under the New Credit Agreement	125,000	125,000		125,000
Revolving credit facility, maturity date	Mar. 21, 2015	Mar. 21, 2015		Mar. 21, 2015
Borrowings under the New Credit Agreement		$ 150,000		$ 150,000
Interest rate under the New Credit Agreement		2.25%		2.25%
Effective interest rate on borrowings		2.70%	1.30%	2.10%	1.40%
Commitment fee under the New Credit Agreement				0.30%