Consolidated Statements of Changes in Shareholders' Equity Statement - USD ($) $ in Millions	Total	Common Shares, $1.00 Par Value	Paid-in Capital	Retained Earnings	AOCI Attributable to Parent [Member]	Serial Preferred Shares [Member]	Voting Preference Shares [Member]
Balance, Beginning of period at Dec. 31, 2015		$ 583.6	$ 1,218.8	$ 4,686.6	$ 800.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Progressive	$ 647.8			647.8
Tax benefit from vesting of equity-based compensation			8.0
Treasury shares purchased		(5.2)	(11.4)	(147.0)
Cash dividends declared on common shares				0.2
Net restricted equity awards issued/vested		2.4	(2.4)
Amortization of equity-based compensation			64.6
Reinvested dividends on restricted stock units			1.1	(1.1)
Adjustment to carrying amount of redeemable noncontrolling interest	(6.0)		6.0
Attributable to noncontrolling interest	(2.3)				(2.3)
Other comprehensive income					248.0
Other, net				(3.4)
Balance, End of period at Sep. 30, 2016	8,094.7	580.8	1,284.7	5,183.1	1,046.1
Balance, Beginning of period at Dec. 31, 2015		583.6	1,218.8	4,686.6	800.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment to carrying amount of redeemable noncontrolling interest	(4.2)
Attributable to noncontrolling interest	3.2
Balance, End of period at Dec. 31, 2016	$ 7,957.1	579.9	1,303.4	5,140.4	933.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock, shares authorized (in shares)						20,000,000.0	5,000,000.0
Preferred stock, shares issued (in shares)						0	0
Preferred stock, shares outstanding (in shares)						0
Common Shares, par value (USD per share)	$ 1.00
Common Shares, par value (USD per share)	$ 1.00
Net income attributable to Progressive	$ 1,015.9			1,015.9
Tax benefit from vesting of equity-based compensation			0.0
Treasury shares purchased		(1.5)	(3.4)	(57.2)
Cash dividends declared on common shares				0.0
Net restricted equity awards issued/vested		3.2	(3.2)
Amortization of equity-based compensation			74.3
Reinvested dividends on restricted stock units			0.3	(0.3)
Adjustment to carrying amount of redeemable noncontrolling interest	6.3		(6.3)
Attributable to noncontrolling interest	(2.9)				(2.9)
Other comprehensive income					295.7
Other, net				17.7
Balance, End of period at Sep. 30, 2017	$ 9,289.4	$ 581.6	$ 1,365.1	$ 6,116.5	$ 1,226.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock, shares authorized (in shares)						20,000,000.0	5,000,000.0
Preferred stock, shares issued (in shares)						0	0
Preferred stock, shares outstanding (in shares)						0
Common Shares, par value (USD per share)	$ 1.00