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Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Deferred tax assets:    
Net operating loss carryforwards $ 117,028 $ 176,490
Research and development credit carryforwards 25,061 22,128
Capitalized research and development 17,195 22,575
Deferred compensation 7,011 8,242
Other 3,116 3,184
Total deferred tax assets 169,411 232,619
Valuation allowance (169,332) (228,794)
Net deferred tax assets 79 3,825
Deferred tax liabilities:    
Unrealized gain on investments   3,825
Amortization of goodwill 111 150
Total deferred tax liabilities $ 111 $ 3,975