Long-Term Debt - Additional Information (Detail) (USD $)	1 Months Ended	9 Months Ended			1 Months Ended		1 Months Ended				9 Months Ended			9 Months Ended		9 Months Ended		9 Months Ended			1 Months Ended		1 Months Ended	9 Months Ended								9 Months Ended
	Jun. 12, 2008	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Jan. 09, 2012 Sun Trust Term Loan [Member]	Sep. 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]	Sep. 30, 2013 Sun Trust Revolver [Member]	Sep. 30, 2013 JPM Term Loan [Member]	Sep. 30, 2013 Wells Fargo Revolver [Member]	Jun. 30, 2013 Wells Fargo Revolver [Member] Beecher Acquisition [Member]	Sep. 30, 2013 Bank Of America Loan Term Loan [Member]	Jun. 30, 2013 Bank Of America Loan Term Loan [Member] Beecher Acquisition [Member]	Sep. 30, 2013 Master Agreement [Member]	Dec. 31, 2012 Master Agreement [Member]	Sep. 30, 2013 Master Agreement [Member] Series C [Member]	Sep. 30, 2013 Master Agreement [Member] Series D [Member]	Sep. 30, 2013 Master Agreement [Member] Series E [Member]	Oct. 12, 2012 New Master Agreement [Member]	Sep. 30, 2013 New Master Agreement [Member]	Jun. 12, 2008 Line of Credit [Member]	Sep. 30, 2013 Line of Credit [Member]	Sep. 30, 2013 Line of Credit [Member] Sun Trust Revolver [Member]	Sep. 30, 2013 Line of Credit [Member] Wells Fargo Revolver [Member]	Sep. 30, 2013 Line of Credit [Member] Bank Of America Loan Term Loan [Member]	Sep. 30, 2013 LIBOR [Member] Minimum [Member]	Sep. 30, 2013 LIBOR [Member] Maximum [Member]	Sep. 30, 2013 Private Placement [Member]	Dec. 31, 2012 Private Placement [Member]	Sep. 30, 2013 Private Placement [Member] Series A [Member]	Sep. 30, 2013 Private Placement [Member] Series B [Member]	Sep. 30, 2013 Private Placement [Member] Senior Notes[Member]
Debt Instrument [Line Items]
Unsecured senior notes outstanding		$ 480,000,000		$ 450,000,000												$ 150,000,000	$ 150,000,000	$ 25,000,000	$ 25,000,000	$ 100,000,000		$ 0								$ 200,000,000	$ 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												Dec. 31, 2016		Dec. 31, 2016				Dec. 22, 2016	Jan. 15, 2015	Sep. 15, 2018					Dec. 31, 2016							Sep. 15, 2011	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 years					3 years
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		100,000,000											75,000,000
Reduction in maximum principal amount		31,863,000	100,000,000																				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		50,000,000
Letter of credit usage fee, minimum																								1.00%		1.00%	1.00%
Letter of credit usage fee, maximum																								1.40%		1.40%	1.40%
LIBOR below base rate																								1.00%		1.00%
Availability fees, minimum																								0.175%		0.175%
Availability fees, maximum																								0.25%		0.25%
Interest rate, minimum																								1.00%		1.00%
Interest rate, maximum																								1.40%		1.40%
Adjusted LIBOR Rate											1.00%
Revolving line of credit													50,000,000
Revolving and term loan														30,000,000	30,000,000
Revolving credit facility remaining amount												30,000,000
Borrowings outstanding												$ 0		$ 30,000,000
Interest margin																											1.50%
30-day LIBOR Rate																												0.18%	0.18%
30-day Adjusted LIBOR Rate																												0.19%	0.19%