Cash Cash Equivalents and Short-term Investments (Detail) (USD $) | 9 Months Ended | 12 Months Ended | | |
In Thousands, unless otherwise specified | Sep. 30, 2013 | Dec. 31, 2012 | Sep. 30, 2012 | Dec. 31, 2011 |
Schedule of Available-for-sale Securities [Line Items] | ' | ' | ' | ' |
Total cash and cash equivalents, Amortized Cost | $174,674 | $76,568 | $110,277 | $65,983 |
Total short-term investments, Amortized Cost | 103,481 | 32,885 | ' | ' |
Total short-term investments, Unrealized Gains | 169 | 9 | ' | ' |
Total short-term investments, Unrealized Losses | -46 | -20 | ' | ' |
Total short-term investments | 103,604 | 32,874 | ' | ' |
Cash, cash equivalents and short-term investments, Amortized Cost | 278,155 | 109,453 | ' | ' |
Cash, cash equivalents and short-term investments, Unrealized Gains | 169 | 9 | ' | ' |
Cash, cash equivalents and short-term investments, Unrealized Losses | -46 | -20 | ' | ' |
Cash, cash equivalents and short-term investments, Estimated Fair Value | 278,278 | 109,442 | ' | ' |
Corporate bonds | ' | ' | ' | ' |
Schedule of Available-for-sale Securities [Line Items] | ' | ' | ' | ' |
Total short-term investments, Amortized Cost | 39,518 | 13,389 | ' | ' |
Total short-term investments, Unrealized Gains | 57 | 2 | ' | ' |
Total short-term investments, Unrealized Losses | -20 | -14 | ' | ' |
Total short-term investments | 39,555 | 13,377 | ' | ' |
U.S. agency securities | ' | ' | ' | ' |
Schedule of Available-for-sale Securities [Line Items] | ' | ' | ' | ' |
Total short-term investments, Amortized Cost | 32,235 | 11,280 | ' | ' |
Total short-term investments, Unrealized Gains | 45 | 4 | ' | ' |
Total short-term investments, Unrealized Losses | -6 | -1 | ' | ' |
Total short-term investments | 32,274 | 11,283 | ' | ' |
Asset-backed securities | ' | ' | ' | ' |
Schedule of Available-for-sale Securities [Line Items] | ' | ' | ' | ' |
Total short-term investments, Amortized Cost | 13,968 | 4,670 | ' | ' |
Total short-term investments, Unrealized Gains | 9 | 1 | ' | ' |
Total short-term investments, Unrealized Losses | -20 | -5 | ' | ' |
Total short-term investments | 13,957 | 4,666 | ' | ' |
U.S. Treasury securities | ' | ' | ' | ' |
Schedule of Available-for-sale Securities [Line Items] | ' | ' | ' | ' |
Total short-term investments, Amortized Cost | 17,760 | 3,546 | ' | ' |
Total short-term investments, Unrealized Gains | 58 | 2 | ' | ' |
Total short-term investments, Unrealized Losses | 0 | 0 | ' | ' |
Total short-term investments | 17,818 | 3,548 | ' | ' |
Cash | ' | ' | ' | ' |
Schedule of Available-for-sale Securities [Line Items] | ' | ' | ' | ' |
Total cash and cash equivalents, Amortized Cost | 31,706 | 59,568 | ' | ' |
Total cash and cash equivalents, Unrealized Gains | 0 | 0 | ' | ' |
Total cash and cash equivalents, Unrealized Losses | 0 | 0 | ' | ' |
Total cash and cash equivalents, Estimated Fair Value | 31,706 | 59,568 | ' | ' |
Money market mutual funds | ' | ' | ' | ' |
Schedule of Available-for-sale Securities [Line Items] | ' | ' | ' | ' |
Total cash and cash equivalents, Amortized Cost | 142,968 | 17,000 | ' | ' |
Total cash and cash equivalents, Unrealized Gains | 0 | 0 | ' | ' |
Total cash and cash equivalents, Unrealized Losses | 0 | 0 | ' | ' |
Total cash and cash equivalents, Estimated Fair Value | 142,968 | 17,000 | ' | ' |
Total cash and cash equivalents | ' | ' | ' | ' |
Schedule of Available-for-sale Securities [Line Items] | ' | ' | ' | ' |
Total cash and cash equivalents, Amortized Cost | 174,674 | 76,568 | ' | ' |
Total cash and cash equivalents, Unrealized Gains | 0 | 0 | ' | ' |
Total cash and cash equivalents, Unrealized Losses | 0 | 0 | ' | ' |
Total cash and cash equivalents, Estimated Fair Value | $174,674 | $76,568 | ' | ' |