Acquisition of Platinum (Intangible Assets) (Details) - USD ($) $ in Thousands		34 Months Ended
	Mar. 02, 2015	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Net identifiable intangible assets at December 31, 2017 related to the acquisition of Platinum		$ 54,471	$ 65,458
Insurance licenses
Business Acquisition [Line Items]
Net identifiable intangible assets at December 31, 2017 related to the acquisition of Platinum		10,267	10,267
Value of business acquired
Business Acquisition [Line Items]
Net identifiable intangible assets at December 31, 2017 related to the acquisition of Platinum		0	673
Internally developed and used computer software
Business Acquisition [Line Items]
Net identifiable intangible assets at December 31, 2017 related to the acquisition of Platinum		0	0
Other non-contractual relationships
Business Acquisition [Line Items]
Net identifiable intangible assets at December 31, 2017 related to the acquisition of Platinum		43,825	53,316
Non-compete agreements
Business Acquisition [Line Items]
Net identifiable intangible assets at December 31, 2017 related to the acquisition of Platinum		0	$ 507
Platinum Underwriters Holdings, Ltd.
Business Acquisition [Line Items]
Finite lived intangible assets acquired	$ 75,200
Indefinite lived intangible assets acquired	8,400
Identifiable intangible assets, before amortization, at March 2, 2015	83,600
Amortization (from March 2, 2015 through December 31, 2017)		(39,914)
Net identifiable intangible assets at December 31, 2017 related to the acquisition of Platinum		$ 43,686
Platinum Underwriters Holdings, Ltd. | Insurance licenses
Business Acquisition [Line Items]
Indefinite lived intangible assets acquired	8,400
Platinum Underwriters Holdings, Ltd. | Key non-contractual relationships
Business Acquisition [Line Items]
Finite lived intangible assets acquired	$ 30,400
Useful life	10 years
Platinum Underwriters Holdings, Ltd. | Value of business acquired
Business Acquisition [Line Items]
Finite lived intangible assets acquired	$ 20,200
Useful life	2 years
Platinum Underwriters Holdings, Ltd. | Renewal rights
Business Acquisition [Line Items]
Finite lived intangible assets acquired	$ 15,800
Useful life	15 years
Platinum Underwriters Holdings, Ltd. | Internally developed and used computer software
Business Acquisition [Line Items]
Finite lived intangible assets acquired	$ 3,500
Useful life	2 years
Platinum Underwriters Holdings, Ltd. | Other non-contractual relationships
Business Acquisition [Line Items]
Finite lived intangible assets acquired	$ 2,300
Useful life	3 years
Platinum Underwriters Holdings, Ltd. | Non-compete agreements
Business Acquisition [Line Items]
Finite lived intangible assets acquired	$ 1,900
Useful life	2 years 6 months
Platinum Underwriters Holdings, Ltd. | Trade name
Business Acquisition [Line Items]
Finite lived intangible assets acquired	$ 1,100
Useful life	6 months