Unrecognized tax benefits (Details 6) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 14,914,000	$ 14,914,000	$ 11,206,000
Additions for tax positions of prior years	0	0	3,708,000
Settlements	(14,777,000)	0	0
Balance at end of year	137,000	14,914,000	14,914,000
Unrecognized tax benefits highly certain deductibility		8,400,000
Unrecognized tax benefits that would affect the effective tax rate	155,000	9,000,000
Unrecognized tax benefits, interest and penalties	18,000	2,500,000
Interest expense	1,800,000	1,200,000	740,000
Interest income	540,000	660,000	290,000
Accrued liability for payment of interest	$ 1,800,000	$ 2,800,000