Summary of Activity Under Credit Arrangements, Excluding Acquired Borrowings (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Total borrowings at end of period	$ 100,000	$ 100,000	$ 200,000
Average outstanding borrowings during year	100,000	133,800	200,000
Maximum outstanding borrowings during year	$ 100,000	$ 200,000	$ 200,000
Interest rate at end of period	4.40%	4.40%	0.65%