Financial Risk Management - Market Price Fluctuations - Pre Tax Foreign Exchange (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Disclosure of nature and extent of risks arising from financial instruments [abstract]
Investing activities	$ 105.4	$ (68.0)
Underwriting activities	(16.8)	5.6
Foreign currency contracts	(33.0)	(1.3)
Foreign currency net gains (losses)	$ 55.6	$ (63.7)

Amount before tax of foreign currency transaction realized and unrealized gain (loss) recognized in the income statement.

No definition available.

Name:	ffh_ForeignCurrencyTransactionGainLossBeforeTax1
Namespace Prefix:	ffh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration