Fair Value Measurements - Maturity Dates of Investments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Fair Value Measurements, Maturity Dates Of Investments [Line Items]
Amortized Cost	$ 353,109	$ 328,796
Estimated Fair Value	352,968	329,335
Less Than 1 Year [Member]
Fair Value Measurements, Maturity Dates Of Investments [Line Items]
Amortized Cost	130,430
Estimated Fair Value	130,490
1 to 5 Years [Member]
Fair Value Measurements, Maturity Dates Of Investments [Line Items]
Amortized Cost	211,744
Estimated Fair Value	212,087
6 to 10 Years [Member]
Fair Value Measurements, Maturity Dates Of Investments [Line Items]
Amortized Cost	10,935
Estimated Fair Value	10,391
Thereafter [Member]
Fair Value Measurements, Maturity Dates Of Investments [Line Items]
Amortized Cost	0
Estimated Fair Value	$ 0