Long-Term Debt - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended			1 Months Ended		1 Months Ended				12 Months Ended			12 Months Ended			1 Months Ended		1 Months Ended	12 Months Ended				12 Months Ended
	Jun. 12, 2008	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jan. 09, 2012 Sun Trust Term Loan	Dec. 31, 2012 Sun Trust Term Loan	Jan. 09, 2012 JPM Bridge Facility	Jan. 09, 2012 JPMorgan Chase Bank, N.A.	Jan. 09, 2012 Sun Trust Revolver	Dec. 31, 2012 Sun Trust Revolver	Dec. 31, 2012 JPM Term Loan	Dec. 31, 2012 Private Placement	Dec. 31, 2011 Private Placement	Dec. 31, 2012 Private Placement Series A	Dec. 31, 2012 Private Placement Series B	Dec. 31, 2012 Private Placement SeniorNotes	Oct. 12, 2012 New Master Agreement Y	Dec. 31, 2012 New Master Agreement	Jun. 12, 2008 Line of Credit	Dec. 31, 2012 Line of Credit	Dec. 31, 2012 Line of Credit Sun Trust Revolver	Dec. 31, 2012 Master Agreement Y	Dec. 31, 2011 Master Agreement	Dec. 31, 2012 Master Agreement Series C	Dec. 31, 2012 Master Agreement Series D	Dec. 31, 2012 Master Agreement Series E	Dec. 31, 2012 Libor
Debt Instrument [Line Items]
Unsecured senior notes outstanding		$ 450,000,000	$ 250,000,000									$ 100,000,000	$ 100,000,000	$ 100,000,000	$ 100,000,000	$ 200,000,000		$ 0				$ 150,000,000	$ 150,000,000	$ 25,000,000	$ 25,000,000	$ 100,000,000
Debt instrument, issuance date														Sep. 15, 2004	Jul. 15, 2004									Dec. 22, 2006	Feb. 01, 2008	Sep. 15, 2011
Debt instrument interest rate stated percentage														5.57%	6.08%									5.66%	5.37%	4.50%
Debt instrument, maturity date														Sep. 15, 2011	Jul. 15, 2014						Dec. 31, 2016			Dec. 22, 2016	Jan. 15, 2015	Sep. 15, 2018
Notes redeemed														100,000,000
Maximum shelf facility capacity																	125,000,000					200,000,000
Length of the issuance period of long-term debt (in years)																	3					3
Long-term debt maturity, period length, maximum (in years)																	Maturity dates, not to exceed ten years					Maturity dates, not to exceed ten years
Lending commitment amount																			50,000,000
Potential maximum increase in lending commitment amount																			100,000,000
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
Revolving credit facility, extended maturity date	Jun. 03, 2013
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
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																											0.23%
30-day Adjusted LIBOR Rate																											0.25%
Interest paid		16,090,000	15,571,000	14,491,000
Maturities of long term debt 2013		93,000
Maturities of long term debt 2014		100,000,000
Maturities of long term debt 2015		25,000,000
Maturities of long term debt 2016		225,000,000
Maturities of long term debt 2017 and beyond		0
Maturities of long term debt 2018 and beyond		$ 100,000,000