Unrecognized tax benefits (Details 6) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Balance at beginning of year	$ 11,206,000	$ 9,378,000	$ 6,148,000
Additions for tax positions of prior years	3,708,000	4,172,000	3,230,000
Settlements	0	(2,344,000)	0
Balance at end of year	14,914,000	11,206,000	9,378,000
Unrecognized tax benefits highly certain deductibility	8,400,000	6,600,000
Unrecognized tax benefits that would affect the effective tax rate	8,500,000	6,000,000
Unrecognized tax benefits, interest and penalties	2,000,000	1,400,000
Interest expense	740,000	780,000	2,000,000
Interest income	290,000	1,900,000	20,000
Accrued liability for payment of interest	$ 1,400,000	$ 970,000