Credit Agreement and Senior Unsecured Notes - Additional Information (Detail) - USD ($)		12 Months Ended
	Jun. 02, 2015	Dec. 31, 2015	Dec. 31, 2014
Credit and Loan Agreement [Line Items]
Borrowings under credit agreement		$ 144,000,000	$ 25,000,000
Credit agreement initiation date		Jun. 02, 2015
Senior unsecured revolving credit agreement	$ 225,000,000
Term loan under credit agreement	50,000,000
Extension of credit agreement		1 year
Amount of rate increase in case of default		2.00%
Debt Refinancing [Member]
Credit and Loan Agreement [Line Items]
Senior secured revolving credit agreement			$ 175,000,000
Credit facility agreement, maturity date			Aug. 31, 2015
Maximum [Member]
Credit and Loan Agreement [Line Items]
Borrowings under credit agreement		$ 94,000,000	$ 25,000,000
Interest rate		3.00%
Annual commitment fee on undrawn funds		0.30%
Maximum [Member] | Debt Refinancing [Member]
Credit and Loan Agreement [Line Items]
Borrowings under credit agreement			$ 25,000,000
Interest rate			2.70%
Minimum [Member]
Credit and Loan Agreement [Line Items]
Annual commitment fee on undrawn funds		0.25%
Restated Prudential Note Purchase Agreement [Member]
Credit and Loan Agreement [Line Items]
Borrowings under credit agreement		$ 175,000,000
Amount of rate increase in case of default		2.00%
Senior unsecured note, issuance date		Jun. 02, 2015
Restated Prudential Note Purchase Agreement [Member] | Series A Note [Member]
Credit and Loan Agreement [Line Items]
Fair value of borrowings outstanding		$ 105,800,000
Restated Prudential Note Purchase Agreement [Member] | Series B Notes [Member]
Credit and Loan Agreement [Line Items]
Fair value of borrowings outstanding		76,400,000
Prudential Loan Agreement [Member] | Series A Note [Member]
Credit and Loan Agreement [Line Items]
Senior unsecured note, aggregate amount issued		$ 100,000,000
Senior unsecured note purchase agreement, maturity date		Feb. 28, 2021
Interest rate on agreement		6.00%
Fair value of borrowings outstanding			$ 106,527,000
Prudential Loan Agreement [Member] | Series B Notes [Member]
Credit and Loan Agreement [Line Items]
Senior unsecured note, aggregate amount issued		$ 75,000,000
Senior unsecured note purchase agreement, maturity date		Jun. 30, 2023
Interest rate on agreement		5.35%
Prior Senior Secured Note Purchase Agreement [Member]
Credit and Loan Agreement [Line Items]
Borrowings under credit agreement			$ 100,000,000
Revolving Facility [Member]
Credit and Loan Agreement [Line Items]
Credit facility agreement, maturity date		Jun. 30, 2018
Borrowings under credit agreement		$ 94,000,000
Revolving facility under credit agreement	175,000,000
Option to increase credit facility	75,000,000
Credit facility amount	$ 250,000,000
Senior unsecured note purchase agreement, maturity date		Jun. 30, 2018
Revolving Facility [Member] | Maximum [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		2.25%
Revolving Facility [Member] | Maximum [Member] | LIBOR [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		3.25%
Revolving Facility [Member] | Minimum [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		0.95%
Revolving Facility [Member] | Minimum [Member] | LIBOR [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		1.95%
Borrowing under Credit Lines - Term Loan [Member]
Credit and Loan Agreement [Line Items]
Credit facility agreement, maturity date		Jun. 30, 2020
Borrowings under credit agreement		$ 50,000,000
Term loan under credit agreement		$ 50,000,000
Senior unsecured note purchase agreement, maturity date		Jun. 30, 2020
Borrowing under Credit Lines - Term Loan [Member] | Maximum [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		2.20%
Borrowing under Credit Lines - Term Loan [Member] | Maximum [Member] | LIBOR [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		3.20%
Borrowing under Credit Lines - Term Loan [Member] | Minimum [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		0.90%
Borrowing under Credit Lines - Term Loan [Member] | Minimum [Member] | LIBOR [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		1.90%