Reserve for Claims and Claim Expenses (Liability for Catastrophe Claims) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Catastrophe Claims [Line Items]
Prior years	$ (164,126)	$ (162,447)	$ (143,798)
Other
Liability for Catastrophe Claims [Line Items]
Prior years	(1,110)	(870)	(6,137)
Property
Liability for Catastrophe Claims [Line Items]
Prior years	(104,876)	(93,786)	(87,258)
Property | Actuarial assumption changes
Liability for Catastrophe Claims [Line Items]
Prior years	0	(1,501)	0
Property | Large and Small Catastrophe Events
Liability for Catastrophe Claims [Line Items]
Prior years	(104,876)	(92,285)	(87,258)
Property | Large Catastrophe Event
Liability for Catastrophe Claims [Line Items]
Prior years	(33,926)	(34,280)	(58,009)
Property | Thailand Floods (2011)
Liability for Catastrophe Claims [Line Items]
Prior years	(15,131)	(18,823)	(11,754)
Property | Tohoku Earthquake and Tsunami (2011)
Liability for Catastrophe Claims [Line Items]
Prior years	(7,314)	(5,313)	(5,408)
Property | New Zealand Earthquake (2011)
Liability for Catastrophe Claims [Line Items]
Prior years	1,987	22,754	(3,088)
Property | 2011 International Events
Liability for Catastrophe Claims [Line Items]
Prior years	(20,458)	(1,382)	(20,250)
Property | Storm Sandy (2012)
Liability for Catastrophe Claims [Line Items]
Prior years	(10,849)	(12,503)	(24,232)
Property | April and May U.S. Tornadoes (2011)
Liability for Catastrophe Claims [Line Items]
Prior years	(4,213)	(10,190)	(14,272)
Property | New Zealand Earthquake (2010)
Liability for Catastrophe Claims [Line Items]
Prior years	6,904	1,095	24,692
Property | Other
Liability for Catastrophe Claims [Line Items]
Prior years	(5,310)	(11,300)	(23,947)
Property | Small Catastrophe Event
Liability for Catastrophe Claims [Line Items]
Prior years	(70,950)	(58,005)	(29,249)
Property | U.S. PCS 13/14 Wind and Thunderstorm (2013)
Liability for Catastrophe Claims [Line Items]
Prior years	(6,286)	(1,882)	(4,239)
Property | Tianjin Explosion (2015)
Liability for Catastrophe Claims [Line Items]
Prior years	(5,686)	0	0
Property | U.S. PCS 15 Wind and Thunderstorm (2013)
Liability for Catastrophe Claims [Line Items]
Prior years	(5,648)	418	2,400
Property | U.S. PCS 81 Wind and Thunderstorm (2015)
Liability for Catastrophe Claims [Line Items]
Prior years	(5,098)	0	0
Property | U.S. PCS 70 Wind and Thunderstorm (2012)
Liability for Catastrophe Claims [Line Items]
Prior years	(3,772)	(1,220)	13,362
Property | U.S. PCS 24 Wind and Thunderstorm (2013)
Liability for Catastrophe Claims [Line Items]
Prior years	(229)	(809)	(6,712)
Property | European Floods (2013)
Liability for Catastrophe Claims [Line Items]
Prior years	(40)	(2,466)	(8,496)
Property | Other
Liability for Catastrophe Claims [Line Items]
Prior years	(44,191)	(52,046)	(25,564)
Casualty and Specialty
Liability for Catastrophe Claims [Line Items]
Prior years	(58,140)	(67,791)	(50,403)
Casualty and Specialty | Actuarial methods - actual reported claims less than expected claims
Liability for Catastrophe Claims [Line Items]
Prior years	(52,601)	(72,551)	(50,403)
Casualty and Specialty | Actuarial assumption changes
Liability for Catastrophe Claims [Line Items]
Prior years	$ (5,539)	$ 4,760	$ 0