Borrowing Arrangements - Narrative (Details) - USD ($)		6 Months Ended	12 Months Ended
	Feb. 20, 2015	Jun. 30, 2019	Dec. 31, 2018	May 31, 2019	Nov. 30, 2018
Debt Instrument [Line Items]
Long-term borrowings		$ 372,300,000	$ 386,700,000
Notional Amount		$ 222,300,000		$ 222,300,000
Derivative, cap interest rate		2.50%
Redemption amount outstanding		$ 1,100,000	1,100,000
Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 400,000,000.0				$ 550,000,000.0
Percentage principal installments		1.25%
Senior credit facility, net of discount
Debt Instrument [Line Items]
Long-term borrowings		$ 354,200,000	$ 371,200,000
Interest rate at period end		4.20%	4.30%
Senior credit facility, net of discount | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	200,000,000.0	$ 174,800,000			325,000,000.0
Letters of credit outstanding		58,100,000
Long-term borrowings		$ 357,500,000
Senior credit facility, net of discount | Senior Credit Facility
Debt Instrument [Line Items]
Interest rate at period end		3.80%	4.00%
Letter of credit facility | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	100,000,000.0				100,000,000.0
Swingline loans | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	20,000,000.0
Term loan facility | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 200,000,000.0				$ 225,000,000.0
Convertible Subordinated Debt
Debt Instrument [Line Items]
Redemption price upon stated maturity (in dollars per share)		$ 19.18
LIBOR Loans | Senior credit facility, net of discount | Restated Credit Agreement
Debt Instrument [Line Items]
Period of total debt to EBITDA ratio	12 months
Base rate loans | Senior credit facility, net of discount | Restated Credit Agreement | Federal funds
Debt Instrument [Line Items]
Interest rate margin on variable rate basis	0.50%
Base rate loans | Senior credit facility, net of discount | Restated Credit Agreement | LIBOR
Debt Instrument [Line Items]
Interest rate margin on variable rate basis	1.00%
Maximum | Senior credit facility, net of discount | Restated Credit Agreement
Debt Instrument [Line Items]
Total debt to EBITDA ratio that is required to be maintained	4.25
Minimum | Senior credit facility, net of discount | Restated Credit Agreement
Debt Instrument [Line Items]
Fixed charge coverage ratio that is required to be maintained	3.50