Borrowings (Details) - USD ($) $ in Millions				3 Months Ended	12 Months Ended
	Nov. 20, 2012	Dec. 09, 2010	Aug. 18, 2006	Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Oct. 20, 2011
Borrowings
Total long-term debt					$ 748.6	$ 749.8
Total long-term debt, net of current portion					598.7	749.8
Cash payments for interest					35.1	35.2	$ 35.0
Interest income					0.7	1.5	0.5
Issuance costs including underwriter's, credit rating agencies' and attorneys' fees and other costs, which are included in other long-term assets							$ 0.6
Outstanding issued letters of credit					30.2
Long-term debt, by maturity
2016					150.0
2020					250.0
2022					350.0
3.75% senior notes due 2022
Borrowings
Total long-term debt					$ 349.2	349.1
Interest rate (as a percent)	3.75%				3.75%
Unamortized discount	$ (1.1)				$ (0.8)	(0.9)
Par value of senior notes	350.0				$ 350.0
Proceeds from issuance of debt	348.9
Issuance costs including underwriter's, credit rating agencies' and attorneys' fees and other costs, which are included in other long-term assets	$ 2.9
Long-term debt redemption price, description of variable discount rate					Treasury Rate
Long-term debt redemption price, basis spread on variable discount rate (as a percent)					0.35%
Percentage of principal amount at which the entity may redeem some or all of the notes prior to specified date					100.00%
Percentage of principal amount at which the entity may redeem some or all of the notes after specified date					100.00%
3.75% senior notes due 2022 | Change-in-control
Borrowings
Percentage of the principal amount at which the notes are redeemable					101.00%
3.75% senior notes due 2022 | Subsidiaries
Borrowings
Subsidiaries indebtedness					$ 0.1
3.75% senior notes due 2022 | Significant Observable Inputs (Level 2)
Borrowings
Fair value of notes					349.3
5.125% senior notes due 2020
Borrowings
Total long-term debt					$ 249.4	249.3
Interest rate (as a percent)		5.125%			5.125%
Unamortized discount		$ 1.1			$ (0.6)	(0.7)
Par value of senior notes		250.0			$ 250.0
Proceeds from issuance of debt		$ 248.9
Long-term debt redemption price, description of variable discount rate					Treasury Rate
Long-term debt redemption price, basis spread on variable discount rate (as a percent)					0.35%
Percentage of principal amount at which the entity may redeem some or all of the notes prior to specified date					100.00%
Percentage of principal amount at which the entity may redeem some or all of the notes after specified date					100.00%
5.125% senior notes due 2020 | Change-in-control
Borrowings
Percentage of the principal amount at which the notes are redeemable					101.00%
5.125% senior notes due 2020 | Significant Observable Inputs (Level 2)
Borrowings
Fair value of notes					$ 268.6
6.125% senior notes due 2016
Borrowings
Total long-term debt					149.9	149.8
Less 6.125% notes due 2016 classified as current					$ (149.9)
Interest rate (as a percent)			6.125%		6.125%
Unamortized discount			$ 1.1		$ (0.1)	(0.2)
Par value of senior notes			150.0		$ 150.0
Proceeds from issuance of debt			$ 148.9
Long-term debt redemption price, description of variable discount rate					Treasury Rate
Long-term debt redemption price, basis spread on variable discount rate (as a percent)					20.00%
Percentage of principal amount at which the entity may redeem some or all of the notes prior to specified date					100.00%
6.125% senior notes due 2016 | Change-in-control
Borrowings
Percentage of the principal amount at which the notes are redeemable					101.00%
6.125% senior notes due 2016 | Significant Observable Inputs (Level 2)
Borrowings
Fair value of notes					$ 152.9
Revolving credit facility
Borrowings
Remaining borrowing capacity					600.0
Maximum borrowing capacity								$ 600.0
Interest on borrowings					0.0	0.0
Industrial development and revenue bonds
Borrowings
Total long-term debt						1.5
Repayment of debt				$ 1.5		1.5
Other, including capital lease obligations
Borrowings
Total long-term debt					0.1	0.1
Uncommitted line of credit
Borrowings
Maximum borrowing capacity					45.0	$ 45.0
Amount outstanding					$ 0.0