Reserves for Loss and Loss Expenses (Reconciliation of Net Reserves) (Details) - USD ($)		3 Months Ended		12 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Insurance [Abstract]
Liability for Unpaid Claims and Claims Adjustment Expense, Net		$ 9,722,040,000	$ 9,372,436,000	$ 9,590,265,000	$ 9,244,872,000
Current Year Claims and Claims Adjustment Expense	[1]	958,684,000	916,649,000
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Adjustment	[2]	(8,727,000)	(5,601,000)
Liability For Unpaid Claims And Claims Adjustment Expense Incurred Loss Reserve Discount Accretion		(12,192,000)	11,273,000
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims		979,603,000	922,321,000
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year		97,461,000	259,633,000
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years		773,571,000	555,884,000
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid		871,032,000	815,517,000
Liability for Unpaid Claims and Claims Adjustment Expense, Foreign Exchange Translation		23,204,000	20,760,000
Reinsurance Recoverables, Gross		1,502,284,000	1,415,906,000
Liability for Claims and Claims Adjustment Expense		11,224,324,000	10,788,342,000	11,197,195,000
Workers' Compensation Discount, Amount		5,761,000		19,072,000
Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims Undiscounted Basis Prior Years, Increase (Decrease)		4,841,000		23,559,000
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ 2,000,000	12,300,000
Reinsurance-Global Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			$ 700,000
Insurance-Domestic Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense				$ 11,600,000

[1] Claims occurring during the current year are net of loss reserve discounts of $5,761,000 and $19,072,000 in 2017 and 2016, respectively.

[2] The increase in estimates for claims occurring in prior years is net of loss reserve discount. On an undiscounted basis, the estimates for claims occurring in prior years increased by $4,841,000 and decreased by $23,559,000 in 2017 and 2016, respectively.