Long-Term Debt - Additional Information (Detail) (USD $)	1 Months Ended	6 Months Ended					6 Months Ended			6 Months Ended	1 Months Ended		1 Months Ended				6 Months Ended		6 Months Ended			1 Months Ended		1 Months Ended	6 Months Ended
	Jun. 12, 2008	Jun. 30, 2012	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Private Placement [Member]	Dec. 31, 2012 Private Placement [Member]	Jun. 30, 2013 Private Placement [Member] Series A [Member]	Jun. 30, 2013 Private Placement [Member] Series B [Member]	Jun. 30, 2013 Private Placement [Member] Senior Notes[Member]	Jun. 30, 2013 JPM Term Loan [Member]	Jan. 09, 2012 Sun Trust Term Loan [Member]	Jun. 30, 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]	Jun. 30, 2013 Sun Trust Revolver [Member]	Jun. 30, 2013 Master Agreement [Member] Y	Dec. 31, 2012 Master Agreement [Member]	Jun. 30, 2013 Master Agreement [Member] Series C [Member]	Jun. 30, 2013 Master Agreement [Member] Series D [Member]	Jun. 30, 2013 Master Agreement [Member] Series E [Member]	Oct. 12, 2012 New Master Agreement [Member] Y	Jun. 30, 2013 New Master Agreement [Member]	Jun. 12, 2008 Line of Credit [Member]	Jun. 30, 2013 Line of Credit [Member]	Jun. 30, 2013 Line of Credit [Member] Sun Trust Revolver [Member]	Jun. 30, 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								$ 150,000,000	$ 150,000,000	$ 25,000,000	$ 25,000,000	$ 100,000,000		$ 0
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. 22, 2016	Jan. 15, 2015	Sep. 15, 2018					Dec. 31, 2016
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 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		100,000,000																						20,000,000
Revolving credit facility, extended maturity date	Jun. 03, 2013
Revolving and term loan										100,000,000	100,000,000		50,000,000	50,000,000	50,000,000
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																											0.19%
30-day Adjusted LIBOR Rate																											0.25%