Notes Payable - Additional Information (Detail) (USD $)	12 Months Ended		12 Months Ended			12 Months Ended				12 Months Ended				3 Months Ended				3 Months Ended		12 Months Ended						12 Months Ended
	Dec. 31, 2013 4.03% Senior Notes due in 2018 [Member]	Apr. 21, 2011 4.03% Senior Notes due in 2018 [Member]	Dec. 31, 2013 4.03% Senior Notes due in 2018 [Member] Minimum [Member]	Dec. 31, 2013 4.03% Senior Notes due in 2018 [Member] Maximum [Member]	Dec. 31, 2013 Unsecured revolving credit facility before Amendment [Member]	Dec. 31, 2013 Amended Credit Facility [Member]	Dec. 31, 2012 Amended Credit Facility [Member]	Dec. 31, 2013 Amended Credit Facility [Member] Unused Fees [Member]	Dec. 31, 2012 Amended Credit Facility [Member] Unused Fees [Member]	Dec. 31, 2013 Amended Credit Facility [Member] Minimum [Member]	Dec. 31, 2013 Amended Credit Facility [Member] Maximum [Member]	Dec. 31, 2013 Amended Credit Facility [Member] LIBOR Loans [Member]	Dec. 31, 2012 Amended Credit Facility [Member] LIBOR Loans [Member]	Dec. 31, 2012 Amended Credit Facility [Member] LIBOR Loans [Member] Minimum [Member]	Dec. 31, 2012 Amended Credit Facility [Member] LIBOR Loans [Member] Maximum [Member]	Dec. 31, 2013 Amended Credit Facility [Member] Base Rate Loans [Member]	Dec. 31, 2012 Amended Credit Facility [Member] Base Rate Loans [Member]	Dec. 31, 2012 Amended Credit Facility [Member] Base Rate Loans [Member] Minimum [Member]	Dec. 31, 2012 Amended Credit Facility [Member] Base Rate Loans [Member] Maximum [Member]	Dec. 31, 2013 Amended Credit Facility [Member] Sweep Service Facility [Member]	Dec. 31, 2013 Letters of Credit [Member]	Jun. 30, 2012 Letters of Credit [Member] Minimum [Member]	Jun. 30, 2012 Letters of Credit [Member] Maximum [Member]	Dec. 31, 2013 Letters of Credit [Member] Sweep Service Facility [Member]	Dec. 31, 2012 Letters of Credit [Member] Sweep Service Facility [Member]	Dec. 31, 2013 Swingline Loans [Member]
Debt Instrument [Line Items]
Revolving credit facility, maturity period						5 years
Revolving credit facility, maturity date						Jun. 15, 2017
Credit facility, maximum					$ 350,000,000	$ 420,000,000														$ 240,000,000		$ 5,000,000	$ 10,000,000
Revolving credit facility, available maximum borrowing capacity						450,000,000														430,000,000
Additional commitments						30,000,000
Unsecured revolving credit facility, sublimit amount																					25,000,000					10,000,000
Amount outstanding						186,000,000	202,000,000													190,000,000				4,000,000	0
Consolidated Fixed Charge Coverage Ratio	2.50					2.50
Consolidated Fixed Charge Coverage Ratio, actual	4.01					4.01
Consolidated Leverage Ratio	2.75					2.75
Consolidated Leverage Ratio, actual	1.81					1.81
Line of credit facility covenants tangible net worth description	Permit Tangible Net Worth, calculated as of the last day of each fiscal quarter, to be less than the sum of (i) $229.0 million, plus (ii) 25% of net income for such fiscal quarter subsequent to December 31, 2010, plus (iii) 90% of the net cash proceeds from the issuance of the Company’s capital stock after December 31, 2010.					Permit Tangible Net Worth as of the end of any fiscal quarter of the Company to be less than the sum of (i) $246,103,400 plus (ii) 25% of the Company’s Consolidated Net Income (as defined in the Amended Credit Facility) (but only if a positive number) for each fiscal quarter ended subsequent to December 31, 2011 plus (iii) 90% of the net cash proceeds from the issuance of the Company’s capital stock after December 31, 2011.
Tangible Net Worth sum	284,800,000		229,000,000			284,800,000				246,103,400
Consolidated Net Income	25.00%					25.00%
Percentage of net cash proceeds from issuance of capital stock	90.00%					90.00%
Actual tangible net worth	362,700,000					362,700,000
Debt instrument description of variable rate basis						(i) LIBOR plus a defined margin, or (ii) the Agent bank’s prime rate (“base rate”) plus a margin. The applicable margin for each type of loan is measured based upon the Consolidated Leverage Ratio at the end of the prior fiscal quarter and ranges from 1.00% to 1.75% for LIBOR loans and 0% to 0.75% for base rate loans. In addition, the Company pays an unused commitment fee for the portion of the $420.0 million credit facility that is not used. These fees are based upon the Consolidated Leverage Ratio and range from 0.15% to 0.30%. As of December 31, 2013 and 2012, the applicable margins were 1.50% for LIBOR based loans, 0.50% for base rate loans and 0.25% for unused fees. Amounts borrowed under the Sweep Service Facility are based upon the Union Bank, N.A. base rate plus an applicable margin and an unused commitment fee for the portion of the $10.0 million facility not used.
Leverage ratio										0.15%	0.30%			1.00%	1.75%			0.00%	0.75%
Unused commitment fees						420,000,000														10,000,000
Debt instrument interest rate at period end								0.25%	0.25%			1.50%	1.50%			0.50%	0.50%
Aggregate principal amount		$ 100,000,000
Senior Notes, interest rate	4.03%	4.03%
Debt instrument, payment terms	Interest on these notes is due semi-annually in arrears and the principal is due in five equal annual installments, with the first payment due on April 21, 2014.
Debt instrument, maturity date	Apr. 21, 2018
Debt instrument, date of first required payment	Apr. 21, 2014
Debt instrument, maturity period				12 years
Debt instrument, average life				10 years