Capital Stock and Earnings Per Share (OMH Only) - Additional Information (Details)		3 Months Ended	9 Months Ended
	Mar. 19, 2020 $ / shares shares	Mar. 19, 2020 USD ($)	Sep. 30, 2020 USD ($) class	Sep. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Number of classes of authorized stock | class			2
Stock repurchase program					$ 200,000,000
Shares repurchased and retired (in shares) | shares	2,031,698
Shares repurchased average price per share (in usd per share) | $ / shares	$ 22.30
Common stock repurchased and retired		$ 45,000,000	$ 45,000,000
Payments of dividends			745,000,000	$ 374,000,000
OMFC
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Payments of dividends			$ 45,000,000