Reconciliation of Total Segment Assets to Total Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting [Abstract]
Total segment assets	$ 4,591,468	$ 3,100,969	$ 3,144,625
Other assets	53,981	48,421
Total assets	$ 4,645,449	$ 3,149,390