Summary of Activity Under Credit Arrangements, Excluding Acquired Borrowings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Debt Disclosure [Abstract]
Total borrowings at December 31	$ 425,707	$ 200,000	$ 280,500
Average outstanding borrowings during year	403,097	361,715	138,400
Maximum outstanding borrowings during year	$ 523,097	$ 500,000	$ 411,000
Interest rate at December 31	2.33%	3.77%	2.98%