Fair Value Measurements and Derivative Instruments (Credit Features) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) derivative	Dec. 31, 2015 USD ($)
Derivative instruments disclosure
Number derivatives that have matured	2
Financial Instruments, Owned and Pledged as Collateral, at Fair Value | $	$ 7.2
Number of Derivatives arriving at their five year anniversary in the Next Twelve Months	2
Maximum cash collateral exposure | $	$ 22.8	$ 14.6
Maturity of at least five years | Lower Limit
Derivative instruments disclosure
Interest rate instrument term	5 years
Interest rate contracts
Derivative instruments disclosure
Number of derivative instruments	7
Standard & Poor's, BBB- Rating | Lower Limit
Derivative instruments disclosure
Credit ratings for senior debt	BBB-
Standard & Poor's, BB Rating
Derivative instruments disclosure
Credit ratings for senior debt	BB+
Moody's, Baa3 Rating
Derivative instruments disclosure
Credit ratings for senior debt	Baa3
Moody's, Ba1 Rating
Derivative instruments disclosure
Credit ratings for senior debt	Ba1