Defined contribution plans (Details 12) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Nonqualified Defined Contribution Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 207,000	$ 104,000	$ 25,000
Defined Contribution Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 36,800,000	$ 34,400,000	$ 33,200,000