Losses and Loss Expenses - Prior Year Development - Reinsurance (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	$ (255,076)	$ (289,889)	$ (315,894)
Reinsurance Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(155,314)	(187,850)	(175,828)
Reinsurance Segment [Member] | Property and other short-tail lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(85,324)	(136,912)	(107,613)
Reinsurance Segment [Member] | Property catastrophe lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	12,400	(60,000)	(16,500)
Reinsurance Segment [Member] | Property other lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(63,400)	(57,300)	(49,000)
Reinsurance Segment [Member] | Marine and aviation lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(34,400)	(19,600)	(42,200)
Reinsurance Segment [Member] | Marine and aviation lines | Release on attritional losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(37,700)
Reinsurance Segment [Member] | Marine and aviation lines | Major catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	3,300
Reinsurance Segment [Member] | Casualty and other
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(69,990)	(50,938)	(68,215)
Reinsurance Segment [Member] | Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(44,300)	(72,200)	(60,300)
Reinsurance Segment [Member] | Other
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(25,700)	21,300	(7,900)
Reinsurance Segment [Member] | Other | Release on attritional losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(22,700)
Reinsurance Segment [Member] | Other | Major catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(3,000)
Reinsurance Segment [Member] | Other | Structured Indemnity
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development			$ (8,400)