Long-term debt (Details) $ / shares in Units, $ in Millions					1 Months Ended		12 Months Ended
	Oct. 15, 2018 USD ($)	Nov. 14, 2017 USD ($) shares	Sep. 29, 2017 Institution	Aug. 12, 2016 shares $ / shares	Dec. 28, 2017 USD ($) shares	Jan. 29, 2014 USD ($) shares $ / shares	Dec. 31, 2018 USD ($) Institution	Oct. 04, 2018 USD ($)	Dec. 31, 2017 USD ($)
Long-term Debt [Abstract]
Debt issuance costs capitalized							$ 13.4		$ 1.4
Long-term indebtedness [Abstract]
Total long-term indebtedness							808.8		13.6
Less current portion of long-term indebtedness, net of debt issuance							(10.1)		0.0
Less net of debt issuance, long-term							(14.2)		(0.1)
Noncurrent portion of long-term indebtedness							784.5		13.5
Maturities of Long-term Debt [Abstract]
2019							11.3
2020							14.1
2021							35.9
2022							33.8
2023 and thereafter							713.7
Total long-term indebtedness							808.8		13.6
Revolving Credit Facility [Member]
Long-Term Debt [Line Items]
Number of lending institutions | Institution			4
Outstanding credit facility								$ 100.0
Long-term Debt [Abstract]
Number of lending institutions | Institution			4
Amount drew down under the facility								$ 100.0
Long-term indebtedness [Abstract]
Total long-term indebtedness							348.0		0.0
Maturities of Long-term Debt [Abstract]
Total long-term indebtedness							348.0		0.0
Term Loan Facility [Member]
Long-term indebtedness [Abstract]
Total long-term indebtedness							447.2		0.0
Maturities of Long-term Debt [Abstract]
Total long-term indebtedness							$ 447.2		0.0
2017 Credit Agreement [Member]
Long-Term Debt [Line Items]
Maturity date							Sep. 29, 2022
Number of lending institutions | Institution							4
Current borrowing capacity							$ 200.0
Borrowing capacity with accordion feature							100.0
Maximum borrowing capacity							$ 300.0
Long-term Debt [Abstract]
Number of lending institutions | Institution							4
Current borrowing capacity							$ 200.0
Maturity date							Sep. 29, 2022
Borrowing capacity with accordion feature							$ 100.0
Maximum borrowing capacity							300.0
2.875% Convertible Senior Notes Due 2021 [Member]
Long-Term Debt [Line Items]
Face amount of debt instrument		$ 250.0			$ 239.4	$ 250.0	$ 10.6
Interest rate percentage		2.875%				2.875%	2.875%
Maturity date						Jan. 15, 2021
Conversion rate of notes per $1,000 principal amount (in shares) | shares		32.386		32.3860	8.5	30.8821
Debt instrument amortization period						7 years
Additional conversion rate of notes per $1,000 principal amount (in shares) | shares		3.1556
Accrued interest on converted notes		$ 2.4
Conversion price per share (in dollars per share) | $ / shares				$ 30.88		$ 32.38
Debt issuance costs					$ 3.6	$ 8.3
Long-term Debt [Abstract]
Maturity date						Jan. 15, 2021
Long-term indebtedness [Abstract]
Total long-term indebtedness							$ 10.6		10.6
Maturities of Long-term Debt [Abstract]
Total long-term indebtedness							10.6		10.6
Amended Credit Agreement [Member] | Minimum [Member]
Long-Term Debt [Line Items]
Commitment fee percentage	0.15%
Long-term Debt [Abstract]
Commitment fee percentage	0.15%
Amended Credit Agreement [Member] | Maximum [Member]
Long-Term Debt [Line Items]
Commitment fee percentage	0.30%
Long-term Debt [Abstract]
Commitment fee percentage	0.30%
Amended Credit Agreement [Member] | Eurocurrency [Member]
Long-Term Debt [Line Items]
Debt instrument, basis spread on variable rate	1.00%
Long-term Debt [Abstract]
Debt instrument, basis spread on variable rate	1.00%
Debt instrument, term of variable rate	1 month
Amended Credit Agreement [Member] | Eurocurrency [Member] | Minimum [Member]
Long-Term Debt [Line Items]
Debt instrument, basis spread on variable rate	1.25%
Long-term Debt [Abstract]
Debt instrument, basis spread on variable rate	1.25%
Amended Credit Agreement [Member] | Eurocurrency [Member] | Maximum [Member]
Long-Term Debt [Line Items]
Debt instrument, basis spread on variable rate	2.00%
Long-term Debt [Abstract]
Debt instrument, basis spread on variable rate	2.00%
Amended Credit Agreement [Member] | Base Rate [Member] | Minimum [Member]
Long-Term Debt [Line Items]
Debt instrument, basis spread on variable rate	0.25%
Long-term Debt [Abstract]
Debt instrument, basis spread on variable rate	0.25%
Amended Credit Agreement [Member] | Base Rate [Member] | Maximum [Member]
Long-Term Debt [Line Items]
Debt instrument, basis spread on variable rate	1.00%
Long-term Debt [Abstract]
Debt instrument, basis spread on variable rate	1.00%
Amended Credit Agreement [Member] | Federal Funds Rate [Member]
Long-Term Debt [Line Items]
Debt instrument, basis spread on variable rate	0.50%
Long-term Debt [Abstract]
Debt instrument, basis spread on variable rate	0.50%
Amended Credit Agreement [Member] | Revolving Credit Facility [Member]
Long-Term Debt [Line Items]
Maturity date	Oct. 13, 2023
Current borrowing capacity	$ 600.0
Debt covenant, leverage ratio, maximum	3.50
Long-term Debt [Abstract]
Current borrowing capacity	$ 600.0
Maturity date	Oct. 13, 2023
Consolidated secured net leverage ratio	2.50
Debt covenant, net leverage ratio	3.50
Amount required for net leverage ratio	$ 200.0
Percentage of debt covenant, net leverage ratio, step up in connection with material acquistions	0.50%
Outstanding credit facility	$ 318.0
Percentage of account receivable from federal government past due	1200.00%
Amended Credit Agreement [Member] | Revolving Credit Facility [Member] | Minimum [Member]
Long-Term Debt [Line Items]
Debt covenant, consolidated debt service coverage ratio, minimum	2.50
Long-term Debt [Abstract]
Debt covenant, consolidated debt service coverage ratio, minimum	2.50
Amended Credit Agreement [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Long-term Debt [Abstract]
Debt covenant, leverage ratio through September 29, 2019	4.00
Debt covenant, leverage ratio from September 30, 2019 to September 29, 2020	3.75
Debt covenant, leverage ratio thereafter	3.50
Debt covenant, leverage ratio adjusted for forth quarter period	4.00
Amended Credit Agreement [Member] | Term Loan Facility [Member]
Long-Term Debt [Line Items]
Current borrowing capacity	$ 450.0
Long-term Debt [Abstract]
Current borrowing capacity	450.0
Outstanding credit facility	$ 450.0
Percentage of original principal amount required to repay in the first two years	2.50%
Percentage of original principal amount required to repay during the third year	5.00%
Percentage of original principal amount required to repay remaining year	7.50%
Cash proceeds excess amount from dispositions of property or casualty events subject to certain reinvestment right	$ 100.0
Other [Member]
Long-term indebtedness [Abstract]
Total long-term indebtedness							3.0		3.0
Maturities of Long-term Debt [Abstract]
Total long-term indebtedness							$ 3.0		$ 3.0