Reserve for Claims and Claim Expenses (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Mar. 02, 2015	Dec. 31, 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	$ 2,568,730	$ 2,632,519	$ 1,345,816		$ 1,462,705
Net claims and claim expenses incurred – prior accident years	(164,126)	(162,447)	(143,798)
Claims and claim expenses recovered	(179,820)	(96,734)	(30,634)
Reinsurance balances payable
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Aggregate deposit liabilities	25,700	32,300
Aggregate deposit assets
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Aggregate deposit assets	0	0
Deposit Contracts, Underwriting Risk Only
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Claims and claim expenses recovered	200	300	300
Deposit Contracts, Timing Risk Only
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Increase (decrease) in other income related to assumed reinsurance contracts	6,200	(6,200)	(100)
Property
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	584,036
Net claims and claim expenses incurred – prior accident years	(104,876)	(93,786)	(87,258)
Property | Actuarial assumption changes
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	0	(1,501)	0
Property | Large Catastrophe Event
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	(33,926)	(34,280)	(58,009)
Property | Thailand Floods (2011)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	(15,131)	(18,823)	(11,754)
Property | Storm Sandy (2012)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	(10,849)	(12,503)	(24,232)
Property | Tohoku Earthquake and Tsunami (2011)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	(7,314)	(5,313)	(5,408)
Property | New Zealand Earthquake (2011)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	1,987	22,754	(3,088)
Property | April and May U.S. Tornadoes (2011)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	(4,213)	(10,190)	(14,272)
Property | New Zealand Earthquake (2010)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	6,904	1,095	24,692
Property | Small Catastrophe Event
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	(70,950)	(58,005)	(29,249)
Property | Tianjin Explosion (2015)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	(5,686)	0	0
Property | European Floods (2013)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	(40)	(2,466)	(8,496)
Property | U.S. PCS 24 Wind and Thunderstorm (2013)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	(229)	(809)	(6,712)
Property | U.S. PCS 70 Wind and Thunderstorm (2012)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	(3,772)	(1,220)	13,362
Property | Other Large and Small Catastrophe Events
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	(21,000)	(69,300)	(51,400)
Property | 2011 International Events
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	(20,458)	(1,382)	(20,250)
Casualty and Specialty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	1,982,580
Net claims and claim expenses incurred – prior accident years	(58,140)	(67,791)	(50,403)
Casualty and Specialty | Actuarial methods - actual reported claims less than expected claims
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	(52,601)	(72,551)	(50,403)
Casualty and Specialty | Actuarial assumption changes
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net claims and claim expenses incurred – prior accident years	$ (5,539)	$ 4,760	$ 0
Platinum Underwriters Holdings, Ltd.
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses				$ 1,400,000
Platinum Underwriters Holdings, Ltd. | Property
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses				179,700
Platinum Underwriters Holdings, Ltd. | Casualty and Specialty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses				$ 1,200,000