Debt - Derivatives (Details) $ in Millions	Apr. 30, 2019 USD ($) derivative	Mar. 31, 2019 USD ($)
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%
Interest rate swap
Interest rate swaps
Interest rate swap agreement, notional amount		$ 400
Interest rate swap | Subsequent event
Interest rate swaps
Interest rate swap agreement, notional amount	$ 350
Terminated interest rate swaps | Subsequent event
Interest rate swaps
Interest rate swap agreement, notional amount	$ 50
Number of derivatives held | derivative	2