Long-Term Debt Letters of Credit (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Bilateral Letter of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End	1.75%
Letters of credit outstanding	$ 55,409
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End	2.25%
Letters of credit outstanding	$ 39,385