Borrowing Arrangements - Narrative (Details)		3 Months Ended	12 Months Ended
	Feb. 20, 2015 USD ($)	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($)	Nov. 30, 2018 USD ($)
Debt Instrument [Line Items]
Long-term borrowings		$ 384,000,000	$ 386,700,000
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
Percentage principal installments		1.25%
Senior credit facility, net of discount
Debt Instrument [Line Items]
Long-term borrowings		$ 368,700,000	$ 371,200,000
Interest rate at period end		4.30%	4.30%
Senior credit facility, net of discount | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	200,000,000.0	$ 174,800,000.0		325,000,000
Letters of credit outstanding		51,300,000
Long-term borrowings		$ 372,200,000
Senior credit facility, net of discount | Senior Credit Facility
Debt Instrument [Line Items]
Interest rate at period end		4.00%	4.00%
Letter of credit facility | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	100,000,000.0			100,000,000
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
Convertible Subordinated Debt
Debt Instrument [Line Items]
Redemption price upon stated maturity (in dollars per share) | $ / shares		$ 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