Reserve for Losses and Loss Adjustment Expenses - Prior year development (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (69,401)	$ (85,789)	$ (152,607)	$ (141,818)
Underwriting segments | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (39,500)	$ (69,800)	$ (96,800)	$ (117,200)
Percentage of prior year development	12.60%	24.00%	17.30%	21.20%
Underwriting segments | Reinsurance | Short Tailed Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (28,100)	$ (48,900)	$ (68,900)	$ (85,400)
Underwriting segments | Reinsurance | Short Tailed Lines | Property Catastrophe And Property Other Than Property Catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	(16,900)	(39,500)	(51,000)	(69,400)
Underwriting segments | Reinsurance | Medium Tailed And Long Tailed Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	(11,400)	(20,900)	(27,900)	(31,800)
Underwriting segments | Reinsurance | Medium Tailed And Long Tailed Lines | Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years		(22,800)	(15,600)
Underwriting segments | Reinsurance | Medium Tailed And Long Tailed Lines | Casualty | 2002-2004
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	(9,000)
Underwriting segments | Reinsurance | Medium Tailed And Long Tailed Lines | Marine And Aviation
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	(2,400)		(12,300)
Underwriting segments | Reinsurance | Long Tailed Lines | Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years				(37,000)
Underwriting segments | Reinsurance | Long Tailed Lines | Marine And Aviation
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years				(3,900)
Underwriting segments | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (2,000)	$ (4,900)	$ (4,100)	$ (11,100)
Percentage of prior year development	0.40%	0.90%	0.40%	1.10%
Underwriting segments | Insurance | Short Tailed Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (5,300)	$ (6,500)	$ (7,200)	$ (10,200)
Underwriting segments | Insurance | Short Tailed Lines | Named catastrophic events
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years				(7,300)
Underwriting segments | Insurance | Medium Tailed Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	(17,800)	9,700	9,700	17,100
Underwriting segments | Insurance | Medium Tailed Lines | Professional Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	(12,100)
Underwriting segments | Insurance | Medium Tailed Lines | Surety
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	(3,600)
Underwriting segments | Insurance | Medium Tailed Lines | Programs
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years		16,400	26,400	22,400
Underwriting segments | Insurance | Medium Tailed Lines | Other product lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years		(6,700)	(16,700)	(5,300)
Underwriting segments | Insurance | Medium Tailed Lines | Construction
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years			13,400
Underwriting segments | Insurance | Medium Tailed And Long Tailed Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	3,300
Underwriting segments | Insurance | Medium Tailed And Long Tailed Lines | Programs
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	12,200
Underwriting segments | Insurance | Medium Tailed And Long Tailed Lines | Construction
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	8,900
Underwriting segments | Insurance | Long Tailed Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years		(8,100)	(6,600)	(18,000)
Underwriting segments | Insurance | Long Tailed Lines | Named catastrophic events
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years		(4,100)
Underwriting segments | Mortgage
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (29,800)	$ (11,100)	$ (53,400)	$ (13,800)
Percentage of prior year development	11.50%	16.60%	10.60%	10.80%
Underwriting segments | Mortgage | Most origination years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (4,900)
Underwriting segments | Mortgage | Other product lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years			$ (13,100)
Underwriting segments | Other
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 1,900		$ 1,700	$ 200