Loss and Loss Adjustment Expense Reserves (Details) - USD ($)	6 Months Ended	9 Months Ended
	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves		$ 13,353,300,000	$ 11,228,200,000	$ 11,368,000,000	$ 10,039,000,000
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments and Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item		2,624,700,000	1,834,200,000	1,801,000,000	1,442,700,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net		10,728,600,000	9,394,000,000	9,567,000,000	8,596,300,000
Current Year Claims and Claims Adjustment Expense		13,886,300,000	12,577,500,000
Prior Year Claims and Claims Adjustment Expense		(42,500,000)	22,900,000
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims		13,928,800,000	12,554,600,000
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year		8,379,400,000	7,768,200,000
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years		4,387,800,000	3,986,200,000
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid		12,767,200,000	11,754,400,000
Liability for Unpaid Claims and Claims Adjustment Expense, Business Acquisitions	$ 2,500,000	0	2,500,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net, Adjusted Beginning Balance				$ 9,567,000,000	$ 8,593,800,000
Accident year 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(51,000,000)
Short-duration Insurance Contracts, Accident Years 2014 and Prior [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		8,000,000
Accident year 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			30,000,000
Accident year 2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			(26,000,000)
Personal Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			9,000,000
Personal Lines | Personal auto businesses
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(76,000,000)
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ 24,000,000	46,000,000
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			$ (31,000,000)