- Summary of Activity Under Credit Arrangements, Excluding Acquired Borrowings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Debt Disclosure [Abstract]
Total borrowings at December 31	$ 1,112,142	$ 425,707	$ 200,000
Average outstanding borrowings during year	638,614	403,097	361,715
Maximum outstanding borrowings during year	$ 1,234,337	$ 523,097	$ 500,000
Interest rate at December 31	2.25%	2.33%	3.77%