Consolidated Statements of Changes in Shareholders' Equity Statement - USD ($) $ in Millions		Total	Preferred Shares, No Par Value	Common Shares, $1.00 Par Value	Paid-in Capital	Retained Earnings	AOCI Attributable to Parent [Member]	Series B Preferred Stock [Member]	Voting Preference Shares [Member]
Balance, Beginning of period at Dec. 31, 2016			$ 0.0	$ 579.9	$ 1,303.4	$ 5,140.4	$ 933.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, New Issues			0.0
Net income attributable to Progressive		$ 791.9				791.9
Treasury shares purchased				(0.7)	(1.5)	(21.9)
Stock Issued During Period, Value, Restricted Stock Award, Gross				1.8	(1.8)
Amortization of equity-based compensation					52.2
Reinvested dividends on restricted stock units					0.1	(0.1)
Adjustment to carrying amount of redeemable noncontrolling interest		1.4			(1.4)
Attributable to noncontrolling interest		(2.2)					(2.2)
Other comprehensive income							219.5
Other, net						(1.5)
Balance, End of period at Jun. 30, 2017		8,991.5	0.0	581.0	1,351.0	5,908.8	1,150.7
Balance, Beginning of period at Dec. 31, 2016			0.0	579.9	1,303.4	5,140.4	933.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment to carrying amount of redeemable noncontrolling interest		8.4
Attributable to noncontrolling interest		(2.3)
Balance, End of period at Dec. 31, 2017		9,284.8	0.0	581.7	1,389.2	6,031.7	1,282.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of Stranded Amounts in AOCI to Retained Earnings | Accounting Standards Update 2018-02 [Member]		4.3
Net income attributable to Progressive		367.6
Attributable to noncontrolling interest		(1.3)
Balance, End of period at Jun. 30, 2017		$ 8,991.5	0.0	581.0	1,351.0	5,908.8	1,150.7
Preferred Stock, No Par Value								$ 0
Preferred Stock, Shares Authorized (shares)									5,000,000.0
Preferred Stock, Shares Issued (shares)									0
Common Shares, par value (USD per share)		$ 1.00
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-01 [Member]	[1]					0.0	0.0
Reclassification of Stranded Amounts in AOCI to Retained Earnings | Accounting Standards Update 2018-02 [Member]	[1]					0.0	0.0
Common Shares, par value (USD per share)		$ 1.00
Cumulative Effect of New Accounting Principle in Period of Adoption		$ 1,300.2
Reclassification of Stranded Amounts in AOCI to Retained Earnings		4.3
Balance, Beginning of period at Dec. 31, 2017		9,284.8	0.0	581.7	1,389.2	6,031.7	1,282.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, New Issues			493.9
Net income attributable to Progressive		1,422.2				1,422.2
Treasury shares purchased				(0.7)	(1.7)	(37.0)
Stock Issued During Period, Value, Restricted Stock Award, Gross				1.5	(1.5)
Amortization of equity-based compensation					37.5
Reinvested dividends on restricted stock units					(0.8)	0.8
Adjustment to carrying amount of redeemable noncontrolling interest		(3.2)			3.2
Attributable to noncontrolling interest		4.6					4.6
Other comprehensive income							(204.2)
Other, net						(1.8)
Balance, End of period at Jun. 30, 2018		11,000.8	493.9	582.5	1,425.9	8,720.4	(221.9)
Preferred Stock, Shares Issued (shares)								500,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Progressive		704.2
Attributable to noncontrolling interest		0.6
Balance, End of period at Jun. 30, 2018		$ 11,000.8	$ 493.9	$ 582.5	$ 1,425.9	8,720.4	(221.9)
Preferred Stock, No Par Value								$ 0
Preferred Stock, Shares Authorized (shares)									5,000,000.0
Preferred Stock, Shares Issued (shares)									0
Common Shares, par value (USD per share)		$ 1.00
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-01 [Member]	[1]					1,300.2	(1,300.2)
Reclassification of Stranded Amounts in AOCI to Retained Earnings | Accounting Standards Update 2018-02 [Member]	[1]					$ 4.3	$ (4.3)

[1] See Note 14 – New Accounting Standards for further discussion.