Investments in affiliates (Investment affiliate rollforward) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Schedule of Equity Method Investments [Line Items]
(Loss) income from investment affiliate	$ (589)	$ 284	$ (4,702)	$ 3,060
Investment affiliate
Schedule of Equity Method Investments [Line Items]
Balance, beginning of period	84,135	85,982	87,673	63,506
Capital contributions	15,732	3,415	16,307	23,115
(Loss) income from investment affiliate	(589)	284	(4,702)	3,060
Balance, end of period	$ 99,278	$ 89,681	$ 99,278	$ 89,681