Fair value measurements Assets and Liabilities on Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy
Transfers in Fair Value Hierarchy	0	0
Assets:
“Available-for-sale” securities	$ 30,817		$ 4,469
Liabilities:
Interest rate swap agreements	287
Fair value measured on recurring basis | Fair Value
Assets:
“Available-for-sale” securities	30,817		4,469
Interest rate swap agreements	958		2,870
Liabilities:
Interest rate swap agreements	1,245		6,191
Fair value measured on recurring basis | Quoted Prices in Active Markets for Identical Assets
Assets:
“Available-for-sale” securities	30,817		4,469
Interest rate swap agreements	0		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	958		2,870
Liabilities:
Interest rate swap agreements	1,245		6,191
Fair value measured on recurring basis | Significant Other Observable Inputs
Assets:
“Available-for-sale” securities	0		0
Interest rate swap agreements	0		0
Liabilities:
Interest rate swap agreements	$ 0		$ 0