Summary of Significant Accounting Policies - Fair Values of the Company's Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 5,900
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	346,266	$ 314,465	$ 257,090	$ 123,532	$ 70,679	$ 131,466	$ 107,916	$ 45,606	$ 34,178
Foreign currency forward contracts						279	630	787	208
Available-for-sale securities	5,941	5,637	6,173	6,047	8,391	12,458
Total assets	352,207	320,102	263,263	129,579	79,070	144,203	108,546	46,393	34,386
Purchase consideration payable	4,967	4,961	4,954	12,443	11,101	24,347	24,003	18,898	14,728
Total liabilities	4,967	4,961	4,954	12,443	11,101	24,347	24,003	18,898	14,728
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	346,266	314,465	257,090	123,532	70,679	131,466	107,916	45,606	34,178
Available-for-sale securities	4,843	4,569	4,991	3,259
Total assets	351,109	319,034	262,081	126,791	70,679	131,466	107,916	45,606	34,178
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency forward contracts						279	630	787	208
Total assets						279	630	787	208
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,098	1,068	1,182	2,788	8,391	12,458
Total assets	1,098	1,068	1,182	2,788	8,391	12,458
Purchase consideration payable	4,967	4,961	4,954	12,443	11,101	24,347	24,003	18,898	14,728
Total liabilities	$ 4,967	$ 4,961	$ 4,954	$ 12,443	$ 11,101	$ 24,347	$ 24,003	$ 18,898	$ 14,728