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

Derivatives and hedging activities - Notional amounts of outstanding interest rate derivatives (Details) € in Thousands, $ in Thousands	Jun. 30, 2015 USD ($)	Jun. 30, 2015 EUR (€)	Dec. 31, 2014 USD ($)	Dec. 31, 2014 EUR (€)
Interest rate swaps
Derivatives and hedging activities
Derivative, notional amount	$ 213,344	€ 74,063	$ 214,206	€ 74,438
Interest rate caps
Derivatives and hedging activities
Derivative, notional amount	$ 17,200		$ 17,200