- Narrative (Details) - Line of Credit Facility - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Jul. 19, 2019	Jul. 18, 2019
Line of Credit Facility [Line Items]
Fair value of net liability for swap positions	$ 4.6	$ 2.4
Variable interest rate	1.80%
Level 2
Line of Credit Facility [Line Items]
Swap agreements at fair value	$ 4.6	$ 2.4
Interest Rate Swap
Line of Credit Facility [Line Items]
Notional amount	$ 200.0
Fixed interest rate	2.70%
Description of variable rate basis	one-month LIBOR
Maturity date	Jun. 30, 2022
Revolving credit facility
Line of Credit Facility [Line Items]
Expiration date	Nov. 18, 2024	Mar. 16, 2022
Commitment fees	$ 1.6	$ 1.4	$ 1.5
Maximum borrowing capacity	500.0
Revolving credit facility | Long-term debt
Line of Credit Facility [Line Items]
Credit facility borrowings	0.0
Letter of Credit sublimit to revolving credit facility
Line of Credit Facility [Line Items]
Letters of credit outstanding	1.6
Letter of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding	70.9
Maximum borrowing capacity	$ 78.0			$ 150.0
AR Facility
Line of Credit Facility [Line Items]
Expiration date	Jun. 30, 2022
Maximum borrowing capacity	$ 125.0				$ 125.0	$ 100.0
AR Facility | Short-term debt
Line of Credit Facility [Line Items]
Credit facility borrowings	$ 105.0	85.0
Interest rate at end of period	2.70%
Remaining borrowing capacity	$ 20.0
Repurchase Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 90.0				$ 90.0	$ 75.0
Repurchase Facility | Short-term debt
Line of Credit Facility [Line Items]
Expiration date	Jun. 30, 2020
Credit facility borrowings	$ 90.0	$ 75.0
Interest rate at end of period	2.90%
Remaining borrowing capacity	$ 0.0
AR Securitization Facilities | Maximum
Line of Credit Facility [Line Items]
Collateral amount	$ 304.7