Fair Value Measures and Other Financial Instruments - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended							12 Months Ended	1 Months Ended
	Jun. 30, 2013	Dec. 31, 2011 Agreement	Jun. 30, 2013 Fixed Rate Debt [Member]	Dec. 31, 2012 Fixed Rate Debt [Member]	Jun. 30, 2013 Variable Rate Debt [Member]	Dec. 31, 2012 Variable Rate Debt [Member]	Jun. 30, 2013 Fair Value, Inputs, Level 2 [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 2 [Member]	Dec. 31, 2011 Three Interest Rate Swap [Member]	Jan. 31, 2012 Fourth Interest Rate Swap [Member]	Jun. 30, 2013 Unsecured Bank Term Loan [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Number of interest rate swap agreements		3
Notional amounts									$ 230,000	$ 70,000
Derivative, fixed interest rate									1.55%	1.50%
LIBOR rate									One-month	One-month
Reduction in blended interest rate											3.24%
Derivative maturity period	2018-01
Net liabilities							3,610	11,710
Accumulated comprehensive income to interest expense, future periods	3,893
Termination payment	3,571
Fair value of debt			827,937	860,217	305,648	298,551
Carrying value of debt			$ 800,604	$ 802,464	$ 300,000	$ 300,000