Fair value measurements Assets and Liabilities on Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Assets:
Available-for-sale securities	$ 4,469	$ 2,709
Fair value measured on recurring basis | Fair Value
Assets:
Available-for-sale securities	4,469	2,709
Interest rate swap agreements	2,870
Liabilities:
Interest rate swap agreements	6,191	20,661
Fair value measured on recurring basis | Quoted Prices in Active Markets for Identical Assets
Assets:
Available-for-sale securities	4,469	2,709
Interest rate swap agreements	0
Liabilities:
Interest rate swap agreements	0	0
Fair value measured on recurring basis | Significant Other Observable Inputs
Assets:
Available-for-sale securities	0	0
Interest rate swap agreements	2,870
Liabilities:
Interest rate swap agreements	6,191	20,661
Fair value measured on recurring basis | Significant Other Observable Inputs
Assets:
Available-for-sale securities	0	0
Interest rate swap agreements	0
Liabilities:
Interest rate swap agreements	$ 0	$ 0