Derivative instruments (Amount and balance sheet location) (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Not designated as hedging instruments | Foreign currency forward contracts
Summary of amount of derivatives designated as hedging instruments
Notional amount		$ 283,765	$ 181,375
Not designated as hedging instruments | Foreign currency forward contracts | Other assets
Summary of amount of derivatives designated as hedging instruments
Fair value, derivative assets	[1]	1,147	2,351
Not designated as hedging instruments | Foreign currency forward contracts | Accounts payable and accrued expenses
Summary of amount of derivatives designated as hedging instruments
Fair value, derivative liabilities	[1]	906	3,421
Not designated as hedging instruments | Interest rate swap contracts
Summary of amount of derivatives designated as hedging instruments
Notional amount		200,000	0
Not designated as hedging instruments | Interest rate swap contracts | Other assets
Summary of amount of derivatives designated as hedging instruments
Fair value, derivative assets	[1]	1,589	0
Not designated as hedging instruments | Interest rate swap contracts | Accounts payable and accrued expenses
Summary of amount of derivatives designated as hedging instruments
Fair value, derivative liabilities	[1]	0	0
Not designated as hedging instruments | Weather derivatives
Summary of amount of derivatives designated as hedging instruments
Notional amount		4,825	0
Not designated as hedging instruments | Weather derivatives | Other assets
Summary of amount of derivatives designated as hedging instruments
Fair value, derivative assets	[1]	853	0
Not designated as hedging instruments | Weather derivatives | Accounts payable and accrued expenses
Summary of amount of derivatives designated as hedging instruments
Fair value, derivative liabilities	[1]	0	0
Designated as hedging instruments | Foreign currency forward contracts
Summary of amount of derivatives designated as hedging instruments
Notional amount		96,293	0
Designated as hedging instruments | Foreign currency forward contracts | Other assets
Summary of amount of derivatives designated as hedging instruments
Fair value, derivative assets	[1]	1,891	0
Designated as hedging instruments | Foreign currency forward contracts | Accounts payable and accrued expenses
Summary of amount of derivatives designated as hedging instruments
Fair value, derivative liabilities	[1]	0	0
Designated as hedging instruments | Interest rate swap contracts
Summary of amount of derivatives designated as hedging instruments
Notional amount		552,263	552,263
Designated as hedging instruments | Interest rate swap contracts | Other assets
Summary of amount of derivatives designated as hedging instruments
Fair value, derivative assets	[1]	9,806	20
Designated as hedging instruments | Interest rate swap contracts | Accounts payable and accrued expenses
Summary of amount of derivatives designated as hedging instruments
Fair value, derivative liabilities	[1]	$ 18,840	$ 1,479

[1] Asset and liability derivatives are classified within other assets and accounts payable and accrued expenses, respectively, within the Company’s Consolidated Balance Sheets.