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Face (par) amount of debt instrument at time of issuance.

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Notes Payable - Additional Information (Detail) - USD ($)			12 Months Ended
	Nov. 05, 2015	Mar. 17, 2014	Dec. 31, 2015	Dec. 31, 2014	Jun. 30, 2012	Apr. 21, 2011
Debt Instrument [Line Items]
Consolidated Fixed Charge Coverage Ratio			2.50%
Consolidated Fixed Charge Coverage Ratio, actual			3.85%
Consolidated Leverage Ratio			2.75%
Consolidated Leverage Ratio, actual			2.32%
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.
Tangible Net Worth sum			$ 304,900,000
Consolidated Net Income			25.00%
Percentage of net cash proceeds from issuance of capital stock			90.00%
Actual tangible net worth			$ 342,400,000
Series A senior notes [Member] \| 4.03% Series A senior notes due in 2018 [Member]
Debt Instrument [Line Items]
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
Senior notes principal balance outstanding			$ 60,000,000	$ 80,000,000
Series B Senior Notes \| 3.68% Series B senior notes due in 2021 [Member]
Debt Instrument [Line Items]
Senior notes interest rate		3.68%	3.68%
Debt instrument, maturity date			Mar. 17, 2021
Senior notes principal balance outstanding			$ 40,000,000
Proceeds from issuance of senior notes		$ 40,000,000
Senior notes description			The Series B Senior Notes are an unsecured obligation of the Company, bear interest at a rate of 3.68% per annum and mature on March 17, 2021. Interest on the Series B Senior Notes is payable semi-annually beginning on September 17, 2014 and continuing thereafter on March 17 and September 17 of each year until maturity.
Senior notes interest payment			Semi-annually
Series C Senior Notes \| 3.84% Series C senior notes due in 2022 [Member]
Debt Instrument [Line Items]
Senior notes interest rate	3.84%		3.84%
Debt instrument, payment terms			Interest on the Series C Senior Notes is payable semi-annually beginning on May 5, 2016 and continuing thereafter on November 5 and May 5 of each year until maturity.
Debt instrument, maturity date			Nov. 05, 2022
Debt instrument, date of first required payment			May 05, 2016
Senior notes principal balance outstanding			$ 60,000,000
Proceeds from issuance of senior notes	$ 60,000,000
Senior notes interest payment			Semi-annually
Minimum [Member]
Debt Instrument [Line Items]
Tangible Net Worth sum			$ 229,000,000
Amended Credit Facility [Member]
Debt Instrument [Line Items]
Revolving credit facility, maturity period			5 years
Revolving credit facility, maturity date			Jun. 15, 2017
Credit facility, maximum			$ 420,000,000
Revolving credit facility, available maximum borrowing capacity			450,000,000
Additional commitments			30,000,000
Amount outstanding			$ 218,000,000	$ 200,000,000
Consolidated Fixed Charge Coverage Ratio			2.50%
Consolidated Fixed Charge Coverage Ratio, actual			3.85%
Consolidated Leverage Ratio			2.32%
Consolidated Leverage Ratio, actual			2.75%
Line of credit facility covenants tangible net worth description			Permit Tangible Net Worth as of the end of any fiscal quarter of the Company to be less than the sum of (i) $246.1 million 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
Tangible Net Worth sum			$ 304,900,000
Consolidated Net Income			25.00%
Percentage of net cash proceeds from issuance of capital stock			90.00%
Actual tangible net worth			$ 342,400,000
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, 2015 and 2014, the applicable margins were 1.75% and 1.50% for LIBOR based loans, respectively, 0.75% and 0.50% for base rate loans, respectively and 0.30% and 0.25% for unused fees, respectively. Amounts borrowed under the Sweep Service Facility are based upon the MUFG 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.
Unused commitment fees			$ 420,000,000
Amended Credit Facility [Member] \| Unused Fees [Member]
Debt Instrument [Line Items]
Debt instrument interest rate at period end			0.30%	0.25%
Amended Credit Facility [Member] \| LIBOR Loans [Member]
Debt Instrument [Line Items]
Debt instrument interest rate at period end			1.75%	1.50%
Amended Credit Facility [Member] \| Base Rate Loans [Member]
Debt Instrument [Line Items]
Debt instrument interest rate at period end			0.75%	0.50%
Amended Credit Facility [Member] \| Sweep Service Facility [Member]
Debt Instrument [Line Items]
Credit facility, maximum			$ 430,000,000
Revolving credit facility, available maximum borrowing capacity			208,600,000
Amount outstanding			221,400,000
Unused commitment fees			10,000,000
Amended Credit Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Tangible Net Worth sum			$ 246,100,000
Leverage ratio			0.15%
Amended Credit Facility [Member] \| Minimum [Member] \| LIBOR Loans [Member]
Debt Instrument [Line Items]
Leverage ratio			1.00%
Amended Credit Facility [Member] \| Minimum [Member] \| Base Rate Loans [Member]
Debt Instrument [Line Items]
Leverage ratio			0.00%
Amended Credit Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Leverage ratio			0.30%
Amended Credit Facility [Member] \| Maximum [Member] \| LIBOR Loans [Member]
Debt Instrument [Line Items]
Leverage ratio			1.75%
Amended Credit Facility [Member] \| Maximum [Member] \| Base Rate Loans [Member]
Debt Instrument [Line Items]
Leverage ratio			0.75%
Unsecured revolving credit facility before Amendment [Member]
Debt Instrument [Line Items]
Credit facility, maximum			$ 350,000,000
Letters of Credit [Member]
Debt Instrument [Line Items]
Credit facility, maximum			25,000,000
Letters of Credit [Member] \| Sweep Service Facility [Member]
Debt Instrument [Line Items]
Amount outstanding			3,400,000	$ 2,500,000
Letters of Credit [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Credit facility, maximum					$ 5,000,000
Letters of Credit [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Credit facility, maximum					$ 10,000,000
Swingline Loans [Member]
Debt Instrument [Line Items]
Credit facility, maximum			$ 10,000,000