Fair value of derivative financial instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Derivatives designated in a cash flow hedging relationship
Fair value of derivative financial instruments
Total	$ (4,890)	$ (8,879)
Derivatives designated in a cash flow hedging relationship \| Interest Rate Swaps \| Accrued liabilities
Fair value of derivative financial instruments
Fair value of liabilities	(3,012)	(3,858)
Derivatives designated in a cash flow hedging relationship \| Interest Rate Swaps \| Other liabilities
Fair value of derivative financial instruments
Fair value of liabilities	(1,878)	(5,021)
Derivatives not designated as hedging instruments
Fair value of derivative financial instruments
Total	2	6
Derivatives not designated as hedging instruments \| Interest Rate Swaps \| Accrued liabilities
Fair value of derivative financial instruments
Fair value of liabilities	0	0
Derivatives not designated as hedging instruments \| Interest Rate Swaps \| Other liabilities
Fair value of derivative financial instruments
Fair value of liabilities	0	0
Derivatives not designated as hedging instruments \| Interest Rate Options \| Other assets
Fair value of derivative financial instruments
Fair value of assets	$ 2	$ 6

Derivatives designated in a cash flow hedging relationship

Derivatives designated in a cash flow hedging relationship | Interest Rate Swaps | Accrued liabilities

Derivatives designated in a cash flow hedging relationship | Interest Rate Swaps | Other liabilities

Derivatives not designated as hedging instruments

Derivatives not designated as hedging instruments | Interest Rate Swaps | Accrued liabilities

Derivatives not designated as hedging instruments | Interest Rate Swaps | Other liabilities

Derivatives not designated as hedging instruments | Interest Rate Options | Other assets