Fair value measurements Assets and Liabilities on Recurring Basis (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
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	$ 137,621	$ 74,265
Interest rate swap agreements	596	0
Fair value measured on recurring basis
Assets:
Available-for-sale securities	137,621	74,265
Interest rate swap agreements	596
Liabilities:
Interest rate swap agreements	4,314	909
Fair value measured on recurring basis | Quoted Prices in Active Markets for Identical Assets
Assets:
Available-for-sale securities	137,621	74,265
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	596
Liabilities:
Interest rate swap agreements	4,314	909
Fair value measured on recurring basis | Significant Unobservable Inputs
Assets:
Available-for-sale securities	0	0
Interest rate swap agreements	0
Liabilities:
Interest rate swap agreements	$ 0	$ 0