Income Taxes (Net Operating and Capital Loss Carryforwards) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net deferred tax assets
Total net deferred income tax assets	$ 215.8	$ 218.3
Change in valuation allowance	(9.1)	19.4	$ 35.7
Alternative minimum tax credit carryforwards, not subject to expiration	2.3
Tax credit carryforwards	20.7	16.9
UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	(4.2)
Net deferred tax assets
Foreign tax credit carryforwards, subject to expiration	0.4
Tax credit carryforwards	18.0
Operating and Capital Loss Carryforward
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	675.5
Net deferred tax assets
Gross deferred tax asset	218.8
Valuation allowance	(144.9)
Total net deferred income tax assets	73.9
Operating and Capital Loss Carryforward | UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	558.7
Net deferred tax assets
Gross deferred tax asset	189.3
Valuation allowance	(115.4)
Total net deferred income tax assets	73.9
Operating and Capital Loss Carryforward | Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	93.8
Net deferred tax assets
Gross deferred tax asset	24.4
Valuation allowance	(24.4)
Total net deferred income tax assets	0.0
Operating and Capital Loss Carryforward | Foreign Tax Authority [Member]
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	19.8
Net deferred tax assets
Gross deferred tax asset	4.6
Valuation allowance	(4.6)
Total net deferred income tax assets	0.0
Operating and Capital Loss Carryforward | UNITED KINGDOM
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	3.2
Net deferred tax assets
Gross deferred tax asset	0.5
Valuation allowance	(0.5)
Total net deferred income tax assets	0.0
No expiration date [Member] | Operating and Capital Loss Carryforward
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	116.8
No expiration date [Member] | Operating and Capital Loss Carryforward | UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
No expiration date [Member] | Operating and Capital Loss Carryforward | Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	93.8
No expiration date [Member] | Operating and Capital Loss Carryforward | Foreign Tax Authority [Member]
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	19.8
No expiration date [Member] | Operating and Capital Loss Carryforward | UNITED KINGDOM
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	3.2
2027-2036 | Operating and Capital Loss Carryforward
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	556.3
2027-2036 | Operating and Capital Loss Carryforward | UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	556.3
2027-2036 | Operating and Capital Loss Carryforward | Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2027-2036 | Operating and Capital Loss Carryforward | Foreign Tax Authority [Member]
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2024-2033 [Member] | Operating and Capital Loss Carryforward | UNITED KINGDOM
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2022-2026 | Operating and Capital Loss Carryforward
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	1.6
2022-2026 | Operating and Capital Loss Carryforward | UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	1.6
2022-2026 | Operating and Capital Loss Carryforward | Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2022-2026 | Operating and Capital Loss Carryforward | Foreign Tax Authority [Member]
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2019-2013 [Member] | Operating and Capital Loss Carryforward | UNITED KINGDOM
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2017-2021 | Operating and Capital Loss Carryforward
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.8
2017-2021 | Operating and Capital Loss Carryforward | UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.8
2017-2021 | Operating and Capital Loss Carryforward | Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2017-2021 | Operating and Capital Loss Carryforward | Foreign Tax Authority [Member]
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2014-2018 [Member] | Operating and Capital Loss Carryforward | UNITED KINGDOM
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
Star & Shield Insurance Exchange [Member]
Net deferred tax assets
Change in valuation allowance	$ (6.9)	$ 1.2