Fair Value - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers between level 1 and level 2	$ 0	$ 0	$ 0	$ 0
Fair value measurements transfers into or out of Level 3				0	0	0
Investments, available-for-sale	135,428		135,428		139,046
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, available-for-sale		$ 0		$ 0	$ 0	$ 0