Debt and Credit Arrangements - Derivatives (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) derivative
Interest rate swaps executed in 2015
Interest rate swaps
Number of derivatives held | derivative	3
Interest rate swaps executed before 2015
Interest rate swaps
Number of derivatives held | derivative	2
Interest rate swap agreement, notional amount	$ 125
Interest rate swaps executed in 2017
Interest rate swaps
Interest rate swap agreement, notional amount	$ 275
Number of additional derivatives executed | derivative	4
Interest rate swap, July 2013, 1.42% fixed
Interest rate swaps
Interest rate swap agreement, notional amount	$ 75
Interest rate swap agreement, fixed interest rate	1.42%
Interest rate swap, December 2014, 1.36% fixed
Interest rate swaps
Interest rate swap agreement, notional amount	$ 50
Interest rate swap agreement, fixed interest rate	1.36%
Interest rate swap, April 2018, 2.33% fixed
Interest rate swaps
Interest rate swap agreement, notional amount	$ 55
Interest rate swap agreement, fixed interest rate	2.33%
Interest rate swap, April 2018, 2.36% fixed
Interest rate swaps
Interest rate swap agreement, notional amount	$ 35
Interest rate swap agreement, fixed interest rate	2.36%
Interest rate swap, April 2018, 2.34% fixed
Interest rate swaps
Interest rate swap agreement, notional amount	$ 35
Interest rate swap agreement, fixed interest rate	2.34%
Interest rate swap, January 2018, 1.99% fixed, $100 million notional amount
Interest rate swaps
Interest rate swap agreement, notional amount	$ 100
Interest rate swap agreement, fixed interest rate	1.99%
Interest rate swap, January 2018, 1.99% fixed, $75 million notional amount
Interest rate swaps
Interest rate swap agreement, notional amount	$ 75
Interest rate swap agreement, fixed interest rate	1.99%
Interest rate swap, January 2018, 2.00% fixed, $75 million notional amount
Interest rate swaps
Interest rate swap agreement, notional amount	$ 75
Interest rate swap agreement, fixed interest rate	2.00%
Interest rate swap, January 2018, 1.99% fixed, $25 million notional amount
Interest rate swaps
Interest rate swap agreement, notional amount	$ 25
Interest rate swap agreement, fixed interest rate	1.99%