Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Pretax total accumulated other comprehensive income (loss), beginning balance	$ (2,179.6)	$ (3,369.6)	$ (2,052.9)	$ (3,556.9)
Pretax total accumulated other comprehensive income (loss), Investment securities	2,012.9	(547.4)	1,514.2	(493.3)
Pretax total accumulated other comprehensive income (loss), Foreign currency translation adjustment	0.1	0.0	(0.2)	0.3
Pretax total accumulated other comprehensive income (loss), before reclassifications	2,013.0	(547.4)	1,514.0	(493.0)
Pretax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities	36.7	(55.6)	(335.3)	(188.3)
Pretax total accumulated other comprehensive income (loss), Interest expense	(0.1)	(0.2)	(0.4)	(0.4)
Total reclassification adjustment for amounts realized in net income, pre tax	36.6	(55.8)	(335.7)	(188.7)
Total other comprehensive income (loss), pre tax	1,976.4	(491.6)	1,849.7	(304.3)
Pretax total accumulated other comprehensive income (loss), ending balance	(203.2)	(3,861.2)	(203.2)	(3,861.2)
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
Total tax (provision) benefit, beginning balance	463.8	715.6	437.2	754.9
Total tax (provision) benefit, Investment securities	(422.7)	114.9	(317.8)	103.5
Total tax (provision) benefit, Foreign currency translation adjustment	0.0	0.0	0.1	(0.1)
Total tax (provision) benefit, before reclassifications	(422.7)	114.9	(317.7)	103.4
Total tax (provision) benefit, Net realized gains (losses) on securities	(7.7)	11.7	70.6	39.5
Total tax (provision) benefit, Interest expense	0.0	0.0	0.1	0.0
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	(7.7)	11.7	70.7	39.5
Total other comprehensive income (loss), tax (provision) benefit	(415.0)	103.2	(388.4)	63.9
Total tax (provision) benefit, ending balance	48.8	818.8	48.8	818.8
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income (loss), beginning balance	(1,715.8)	(2,654.0)	(1,615.7)	(2,802.0)
After tax total accumulated other comprehensive income (loss), Investment securities	1,590.2	(432.5)	1,196.4	(389.8)
After tax total accumulated other comprehensive income (loss), Foreign currency translation adjustment	0.1	0.0	(0.1)	0.2
After tax total accumulated other comprehensive income (loss), before reclassifications	1,590.3	(432.5)	1,196.3	(389.6)
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities	29.0	(43.9)	(264.7)	(148.8)
After tax total accumulated other comprehensive income (loss), Interest expense	(0.1)	(0.2)	(0.3)	(0.4)
Total reclassification adjustment for amounts realized in net income, after tax	28.9	(44.1)	(265.0)	(149.2)
Total other comprehensive income (loss), after tax	1,561.4	(388.4)	1,461.3	(240.4)
After tax total accumulated other comprehensive income (loss), ending balance	(154.4)	(3,042.4)	(154.4)	(3,042.4)
Total net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income (loss), beginning balance	(1,700.9)	(2,638.7)	(1,600.8)	(2,786.3)
After tax total accumulated other comprehensive income (loss), Investment securities	1,590.2	(432.5)	1,196.4	(389.8)
After tax total accumulated other comprehensive income (loss), Foreign currency translation adjustment	0.0	0.0	0.0	0.0
After tax total accumulated other comprehensive income (loss), before reclassifications	1,590.2	(432.5)	1,196.4	(389.8)
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities	29.0	(43.9)	(264.7)	(148.8)
After tax total accumulated other comprehensive income (loss), Interest expense	0.0	0.0	0.0	0.0
Total reclassification adjustment for amounts realized in net income, after tax	29.0	(43.9)	(264.7)	(148.8)
Total other comprehensive income (loss), after tax	1,561.2	(388.6)	1,461.1	(241.0)
After tax total accumulated other comprehensive income (loss), ending balance	(139.7)	(3,027.3)	(139.7)	(3,027.3)
Net unrealized losses on forecasted transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income (loss), beginning balance	(13.8)	(14.3)	(14.0)	(14.5)
After tax total accumulated other comprehensive income (loss), Investment securities	0.0	0.0	0.0	0.0
After tax total accumulated other comprehensive income (loss), Foreign currency translation adjustment	0.0	0.0	0.0	0.0
After tax total accumulated other comprehensive income (loss), before reclassifications	0.0	0.0	0.0	0.0
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities	0.0	0.0	0.0	0.0
After tax total accumulated other comprehensive income (loss), Interest expense	(0.1)	(0.2)	(0.3)	(0.4)
Total reclassification adjustment for amounts realized in net income, after tax	(0.1)	(0.2)	(0.3)	(0.4)
Total other comprehensive income (loss), after tax	0.1	0.2	0.3	0.4
After tax total accumulated other comprehensive income (loss), ending balance	(13.7)	(14.1)	(13.7)	(14.1)
Foreign currency translation adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income (loss), beginning balance	(1.1)	(1.0)	(0.9)	(1.2)
After tax total accumulated other comprehensive income (loss), Investment securities	0.0	0.0	0.0	0.0
After tax total accumulated other comprehensive income (loss), Foreign currency translation adjustment	0.1	0.0	(0.1)	0.2
After tax total accumulated other comprehensive income (loss), before reclassifications	0.1	0.0	(0.1)	0.2
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities	0.0	0.0	0.0	0.0
After tax total accumulated other comprehensive income (loss), Interest expense	0.0	0.0	0.0	0.0
Total reclassification adjustment for amounts realized in net income, after tax	0.0	0.0	0.0	0.0
Total other comprehensive income (loss), after tax	0.1	0.0	(0.1)	0.2
After tax total accumulated other comprehensive income (loss), ending balance	$ (1.0)	$ (1.0)	$ (1.0)	$ (1.0)