Debt - Derivatives (Details) $ in Millions	Sep. 30, 2018 USD ($)
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%