Long-Term Debt - Additional Information (Detail) (USD $)	1 Months Ended					3 Months Ended			1 Months Ended		1 Months Ended				3 Months Ended	1 Months Ended	3 Months Ended		1 Months Ended			3 Months Ended		3 Months Ended
	Jun. 12, 2008	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Private Placement [Member]	Dec. 31, 2012 Private Placement [Member]	Mar. 31, 2013 Private Placement [Member] Series A [Member]	Mar. 31, 2013 Private Placement [Member] Series B [Member]	Mar. 31, 2013 Private Placement [Member] Senior Notes[Member]	Jan. 09, 2012 Sun Trust Term Loan [Member]	Mar. 31, 2013 Sun Trust Term Loan [Member]	Jan. 09, 2012 JPM Bridge Facility [Member]	Jan. 09, 2012 JPMorgan Chase Bank, N.A. [Member]	Jan. 09, 2012 Sun Trust Revolver [Member]	Mar. 31, 2013 Sun Trust Revolver [Member]	Mar. 31, 2013 JPM Term Loan [Member]	Jun. 12, 2008 Line of Credit [Member]	Mar. 31, 2013 Line of Credit [Member]	Mar. 31, 2013 Line of Credit [Member] Sun Trust Revolver [Member]	Oct. 12, 2012 New Master Agreement [Member] Y	Mar. 31, 2013 New Master Agreement [Member]	Dec. 31, 2012 New Master Agreement [Member]	Mar. 31, 2013 Master Agreement [Member] Y	Dec. 31, 2012 Master Agreement [Member]	Mar. 31, 2013 Master Agreement [Member] Series C [Member]	Mar. 31, 2013 Master Agreement [Member] Series D [Member]	Mar. 31, 2013 Master Agreement [Member] Series E [Member]	Mar. 31, 2013 Libor [Member]
Debt Instrument [Line Items]
Unsecured senior notes outstanding		$ 450,000,000	$ 450,000,000	$ 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
Borrowings	0		0							150,000,000				100,000,000		75,000,000				0	0
Lending commitment amount																50,000,000
Potential maximum increase in lending commitment amount																100,000,000
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																											0.20%
30-day Adjusted LIBOR Rate																											0.25%