Derivatives and hedging activities - Notional amounts of outstanding interest rate derivatives (Details) (Interest Rate Swaps) In Thousands, unless otherwise specified	Mar. 31, 2014 USD ($)	Mar. 31, 2014 EUR (€)	Dec. 31, 2013 USD ($)	Dec. 31, 2013 EUR (€)
Derivatives and hedging activities
Derivative, notional amount	$ 172,500	€ 75,000	$ 63,700	€ 137,469

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant