Income Taxes (Net Operating and Capital Loss Carryforwards) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net deferred tax assets
Total net deferred income tax assets	$ 598,100,000	$ 617,600,000
Valuation allowance	18,100,000	33,600,000	14,100,000
Alternative minimum tax credit carryforwards, not subject to expiration	2,000,000
Tax credit carryforwards	25,800,000	23,100,000
UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	32,200,000
Net deferred tax assets
Foreign tax credit carryforwards, subject to expiration	12,700,000
Tax credit carryforwards	11,100,000
Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	983,400,000
Sirius Re Holdings Inc.
Net deferred tax assets
Foreign tax credit carryforwards, subject to expiration	12,100,000
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	2,170,000,000
Net deferred tax assets
Gross deferred tax asset	640,800,000
Valuation allowance	(265,500,000)
Total net deferred income tax assets	375,300,000
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	632,300,000
Net deferred tax assets
Gross deferred tax asset	213,400,000
Valuation allowance	(97,200,000)
Total net deferred income tax assets	116,200,000
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	1,417,500,000
Net deferred tax assets
Gross deferred tax asset	414,200,000
Valuation allowance	(166,100,000)
Total net deferred income tax assets	248,100,000
Operating and Capital Loss Carryforward | SWEDEN
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	106,000,000
Net deferred tax assets
Gross deferred tax asset	9,800,000
Valuation allowance	0
Total net deferred income tax assets	9,800,000
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	5,800,000
Net deferred tax assets
Gross deferred tax asset	1,200,000
Valuation allowance	0
Total net deferred income tax assets	1,200,000
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	7,100,000
Net deferred tax assets
Gross deferred tax asset	1,900,000
Valuation allowance	(1,900,000)
Total net deferred income tax assets	0
Operating and Capital Loss Carryforward | NETHERLANDS
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	1,300,000
Net deferred tax assets
Gross deferred tax asset	300,000
Valuation allowance	(300,000)
Total net deferred income tax assets	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	1,536,400,000
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
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	1,417,500,000
No expiration date [Member] | Operating and Capital Loss Carryforward | SWEDEN
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	106,000,000
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	5,800,000
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	7,100,000
No expiration date [Member] | Operating and Capital Loss Carryforward | NETHERLANDS
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0
2024-2033 [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	621,600,000
2024-2033 [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	621,600,000
2024-2033 [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	0
2024-2033 [Member] | Operating and Capital Loss Carryforward | SWEDEN
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	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
2024-2033 [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	0
2024-2033 [Member] | Operating and Capital Loss Carryforward | NETHERLANDS
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0
2019-2013 [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	10,400,000
2019-2013 [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	9,600,000
2019-2013 [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	0
2019-2013 [Member] | Operating and Capital Loss Carryforward | SWEDEN
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	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
2019-2013 [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	0
2019-2013 [Member] | Operating and Capital Loss Carryforward | NETHERLANDS
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	800,000
2014-2018 [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	1,600,000
2014-2018 [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	1,100,000
2014-2018 [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	0
2014-2018 [Member] | Operating and Capital Loss Carryforward | SWEDEN
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	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
2014-2018 [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	0
2014-2018 [Member] | Operating and Capital Loss Carryforward | NETHERLANDS
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	500,000
2013 [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	0
2013 [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
2013 [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	0
2013 [Member] | Operating and Capital Loss Carryforward | SWEDEN
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0
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
2013 [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	0
2013 [Member] | Operating and Capital Loss Carryforward | NETHERLANDS
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	$ 0