Fair Value Measurement - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Total assets at fair value	$ 36,086
Liabilities:
Total liabilities at fair value	247,459	$ 5,175
Cash equivalents
Assets:
Total assets at fair value	36,086	133,540
Contingent consideration
Liabilities:
Total liabilities at fair value	24,301	5,175
Deferred consideration
Liabilities:
Total liabilities at fair value	223,158	0
Level 1 [Member]
Assets:
Total assets at fair value		133,540
Liabilities:
Total liabilities at fair value	0	0
Level 1 [Member] | Cash equivalents
Assets:
Total assets at fair value	36,086	133,540
Level 1 [Member] | Contingent consideration
Liabilities:
Total liabilities at fair value	0	0
Level 1 [Member] | Deferred consideration
Liabilities:
Total liabilities at fair value	0	0
Level 2 [Member]
Assets:
Total assets at fair value	0	0
Liabilities:
Total liabilities at fair value	223,158	0
Level 2 [Member] | Cash equivalents
Assets:
Total assets at fair value	0	0
Level 2 [Member] | Contingent consideration
Liabilities:
Total liabilities at fair value	0	0
Level 2 [Member] | Deferred consideration
Liabilities:
Total liabilities at fair value	223,158	0
Level 3 [Member]
Assets:
Total assets at fair value	0	0
Liabilities:
Total liabilities at fair value	24,301	5,175
Level 3 [Member] | Cash equivalents
Assets:
Total assets at fair value	0	0
Level 3 [Member] | Contingent consideration
Liabilities:
Total liabilities at fair value	24,301	5,175
Level 3 [Member] | Deferred consideration
Liabilities:
Total liabilities at fair value	$ 0	$ 0