Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Oct. 31, 2011
Assets
Federally-sponsored and corporate debt securities	$ 307,513	$ 420,731
Discounted Cash Flow Valuation Technique [Member] | Maximum
Liabilities
Cost of debt and weighted average cost of capital (as a percent)	15.50%	15.50%
Discounted Cash Flow Valuation Technique [Member] | Minimum
Liabilities
Cost of debt and weighted average cost of capital (as a percent)	6.10%	6.10%
Convertible Notes Due 2016
Liabilities
Stated percentage rate	1.00%		1.00%
Recurring fair value measurements | Level 1
Assets
Money market funds	$ 91,799	$ 298,416
Total assets	91,799	298,416
Liabilities
Convertible notes due 2016	124,012	388,153
Total liabilities	124,012	388,153
Recurring fair value measurements | Level 2
Assets
Federally-sponsored and corporate debt securities	307,513	420,731
Total assets	307,513	420,731
Recurring fair value measurements | Level 3
Liabilities
Contingent consideration	10,255	11,502
Total liabilities	10,255	11,502
Recurring fair value measurements | Balance
Assets
Money market funds	91,799	298,416
Federally-sponsored and corporate debt securities	307,513	420,731
Total assets	399,312	719,147
Liabilities
Convertible notes due 2016	124,012	388,153
Contingent consideration	10,255	11,502
Total liabilities	$ 134,267	$ 399,655