Activity under Polaris Credit Arrangements (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Debt Disclosure [Line Items]
Total borrowings at end of period	$ 100,000	$ 200,000	$ 200,000
Average outstanding borrowings during year	133,800	200,000	268,100
Maximum outstanding borrowings during year	$ 200,000	$ 200,000	$ 345,000
Interest rate at end of period	4.40%	0.65%	0.79%