Consolidated Statements of Changes in Shareholders' Equity Statement - USD ($) $ in Millions	Total	Common Shares, $1.00 Par Value	Paid-in Capital	Retained Earnings	Accumulated other comprehensive income after tax	Serial Preferred Shares [Member]	Voting Preference Shares [Member]
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
Balance, Beginning of period at Dec. 31, 2014		$ 587.8	$ 1,184.3	$ 4,133.4	$ 1,023.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Progressive	$ 937.2			937.2
Tax benefit from vesting of equity-based compensation			15.0
Treasury shares purchased		(6.3)	(12.9)	(155.7)
Net restricted equity awards (issued)/(vested)/forfeited		3.1	(3.1)
Amortization of equity-based compensation			45.9
Reinvested dividends on restricted stock units			(0.1)	0.1
Adjustment to carrying amount of redeemable noncontrolling interest			(31.8)
Cash dividends declared on common shares				0.0
Attributable to noncontrolling interest	1.2				1.2
Other comprehensive income	(255.1)				(255.1)
Other, net				(4.1)
Balance, End of period at Sep. 30, 2015	7,462.0	584.6	1,197.3	4,910.9	769.2
Balance, Beginning of period at Dec. 31, 2014		587.8	1,184.3	4,133.4	1,023.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Tax benefit from vesting of equity-based compensation	16.8
Balance, End of period at Dec. 31, 2015	$ 7,289.4	583.6	1,218.8	4,686.6	800.4
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
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
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)
Net restricted equity awards (issued)/(vested)/forfeited		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
Cash dividends declared on common shares				0.2
Attributable to noncontrolling interest	(2.3)				(2.3)
Other comprehensive income	248.0				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
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