Subsequent Events - Additional Information (Detail) (USD $)	12 Months Ended		12 Months Ended					12 Months Ended	1 Months Ended		12 Months Ended	1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Additional vesting [Member]	Dec. 31, 2012 Performance Period [Member]	Dec. 31, 2012 VSEP awards [Member]	Dec. 31, 2012 Time-based vesting [Member]	Feb. 28, 2013 Subsequent Events [Member] Oxford [Member]	Dec. 31, 2012 Unsecured revolving credit facility with PNC Bank National Association [Member]	Jan. 31, 2013 Unsecured revolving credit facility with PNC Bank National Association [Member] Subsequent Events [Member]	Jan. 31, 2013 Secured Term Loan arranged by KeyBanc Capital Markets and RBC Capital Markets [Member] Subsequent Events [Member]	Dec. 31, 2012 Unsecured revolving credit facility arranged by JP Morgan Securities, LLC and Wells Fargo Securities, LLC [Member]	Jan. 31, 2013 Unsecured revolving credit facility arranged by JP Morgan Securities, LLC and Wells Fargo Securities, LLC [Member] Subsequent Events [Member]
Subsequent Event [Line Items]
Unsecured revolving credit facility								$ 65,000,000	$ 65,000,000
Secured term loan	400,000,000	500,000,000								400,000,000
Debt instrument basis spread on variable rate									1.40%	1.55%		1.40%
Unsecured revolving credit facility, borrowing capacity											750,000,000	750,000,000
Term loan maturity date										Apr. 30, 2017
Accordion feature											1,250,000,000	1,250,000,000
Debt instrument basis spread on variable rate decrease									0.25%	0.15%		0.25%
Facility fee								0.35%	0.30%		0.35%	0.30%
Revolving credit facility maturity period								Feb. 29, 2016	Apr. 30, 2017		Feb. 29, 2016	Apr. 30, 2017
Period for which the options granted are generally vested	3 years		4 years	3 years	7 years	4 years
Amount transferred to pay vendors pursuant to an agreement							11,000,000
Unasserted Claim under Agreement							$ 5,000,000