Long-Term Debt - Additional Information (Detail) (USD $)	9 Months Ended		9 Months Ended										9 Months Ended						1 Months Ended			9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Sun Trust Term Loan [Member]	Sep. 30, 2012 Sun Trust Revolver [Member]	Sep. 30, 2012 JPMorgan Chase Bank, N.A. [Member]	Sep. 30, 2012 JPM Bridge Facility [Member]	Sep. 30, 2012 JPM Term Loan [Member]	Sep. 30, 2012 Line of Credit [Member]	Sep. 30, 2011 Line of Credit [Member]	Sep. 30, 2012 Line of Credit [Member] Sun Trust Revolver [Member]	Sep. 30, 2012 Master Agreement [Member] Y	Dec. 31, 2011 Master Agreement [Member]	Sep. 30, 2012 Master Agreement [Member] Series C [Member]	Sep. 30, 2012 Master Agreement [Member] Series D [Member]	Sep. 30, 2012 Master Agreement [Member] Series E [Member]	Sep. 30, 2012 LIBOR [Member]	Sep. 30, 2012 New Master Agreement [Member]	Dec. 31, 2011 New Master Agreement [Member]	Oct. 31, 2012 New Master Agreement [Member] Subsequent Event [Member] Y	Sep. 30, 2012 Private Placement [Member] Repayment of Debt [Member]	Dec. 31, 2011 Private Placement [Member] Repayment of Debt [Member]	Sep. 30, 2012 Private Placement [Member] Series A [Member]	Sep. 30, 2012 Private Placement [Member] Series B [Member]	Sep. 30, 2011 Private Placement [Member] Senior Notes [Member]
Debt Instrument [Line Items]
Unsecured senior notes outstanding	$ 450,000,000	$ 250,000,000									$ 150,000,000	$ 150,000,000	$ 25,000,000	$ 25,000,000	$ 100,000,000		$ 0	$ 0		$ 100,000,000	$ 100,000,000	$ 100,000,000	$ 100,000,000	$ 200,000,000
Debt instrument, issuance date													Dec. 22, 2006	Feb. 01, 2008	Sep. 15, 2011							Sep. 15, 2004	Jul. 15, 2004
Debt instrument interest rate stated percentage													5.66%	5.37%	4.50%							5.57%	6.08%
Debt instrument, maturity date						Feb. 03, 2012				Dec. 31, 2016			Dec. 22, 2016	Jan. 15, 2015	Sep. 15, 2018								Jul. 15, 2014
Notes redeemed																						100,000,000
Maximum shelf facility capacity											200,000,000								125,000,000
Length of the issuance period of long-term debt (in years)											3								3
Long-term debt, maturities, repayment terms, maximum number of years											ten								Ten years
Lending commitment amount								50,000,000
Potential maximum increase in lending commitment amount								100,000,000
Revolving credit facility, maturity date	Dec. 11, 2011
Revolving credit facility, extended maturity date	June 3, 2013
Revolving credit facility in the maximum principal amount			150,000,000	100,000,000				75,000,000
Reduction in maximum principal amount								20,000,000	20,000,000
Revolving and term loan			100,000,000	50,000,000	50,000,000	50,000,000	100,000,000
Increase in revolving credit facility			$ 50,000,000	$ 50,000,000
Letter of credit usage fee, minimum								1.00%
Letter of credit usage fee, maximum								1.40%
LIBOR below base rate								1.00%
Availability fees, minimum								0.175%
Availability fees, maximum								0.25%
Interest rate, minimum								1.00%
Interest rate, maximum								1.40%
Adjusted LIBOR Rate							1.00%
30-day LIBOR Rate																0.2305%
30-day Adjusted LIBOR Rate																0.25%