Segment Reporting - Adjusted Pretax Operating Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Net gains (losses) on investments and other financial instruments		$ 10,241	$ (80,733)	$ 15,603
Impairment of goodwill (Note 2)	$ (10,000)	(9,802)	0	0
Amortization of other acquired intangible assets		(5,483)	(4,308)	(3,450)
Impairment of other long-lived assets and other non-operating items		(13,082)	(14,850)	(3,561)
Pretax income		767,487	952,779	764,832
Lease-Related Assets
Segment Reporting Information [Line Items]
Impairment of other long-lived assets and other non-operating items		(9,000)
Internal-use software
Segment Reporting Information [Line Items]
Impairment of other long-lived assets and other non-operating items		(5,000)
Operating Segments
Segment Reporting Information [Line Items]
Adjusted pretax operating income (loss)		743,554	1,043,745	754,222
Net gains (losses) on investments and other financial instruments		0	0	1,509
All Other adjusted pretax operating income
Segment Reporting Information [Line Items]
Adjusted pretax operating income (loss)		42,873	8,972	3,527
Net gains (losses) on investments and other financial instruments		814	47	0
Adjustments
Segment Reporting Information [Line Items]
Net gains (losses) on investments and other financial instruments		9,427	(80,780)	14,094
Mortgage Insurance | Operating Segments
Segment Reporting Information [Line Items]
Adjusted pretax operating income (loss)		829,824	1,131,943	781,546
Net gains (losses) on investments and other financial instruments		0	0	0
homegenius | Operating Segments
Segment Reporting Information [Line Items]
Adjusted pretax operating income (loss)		(86,270)	(88,198)	(27,324)
Net gains (losses) on investments and other financial instruments		$ 0	$ 0	$ 1,509