Securities (Securities with Unrealized Losses) (Details)	Sep. 30, 2022 USD ($) Security	Dec. 31, 2021 USD ($)
FAIR VALUE
LESS THAN 12 MONTHS	$ 19,453,200,000	$ 16,660,700,000
12 MONTHS OR LONGER	4,097,400,000	2,037,000,000
TOTAL	23,550,600,000	18,697,700,000
UNREALIZED LOSSES
LESS THAN 12 MONTHS	1,095,300,000	223,600,000
12 MONTHS OR LONGER	188,800,000	25,000,000.0
TOTAL	1,284,100,000	248,600,000
Net amount of recognized liabilities for repurchase agreements	561,400,000	531,900,000
U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	1,198,600,000	239,000,000.0
12 MONTHS OR LONGER	70,600,000	0
TOTAL	1,269,200,000	239,000,000.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	97,200,000	9,400,000
12 MONTHS OR LONGER	2,700,000	0
TOTAL	99,900,000	9,400,000
Obligations of States and Political Subdivisions
FAIR VALUE
LESS THAN 12 MONTHS	743,900,000	1,699,500,000
12 MONTHS OR LONGER	94,000,000.0	31,600,000
TOTAL	837,900,000	1,731,100,000
UNREALIZED LOSSES
LESS THAN 12 MONTHS	108,000,000.0	37,400,000
12 MONTHS OR LONGER	17,500,000	1,200,000
TOTAL	125,500,000	38,600,000
Government Sponsored Agency
FAIR VALUE
LESS THAN 12 MONTHS	8,370,400,000	8,207,300,000
12 MONTHS OR LONGER	2,063,300,000	1,821,400,000
TOTAL	10,433,700,000	10,028,700,000
UNREALIZED LOSSES
LESS THAN 12 MONTHS	239,900,000	90,200,000
12 MONTHS OR LONGER	60,000,000.0	20,000,000.0
TOTAL	299,900,000	110,200,000
Non-U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	152,600,000	230,000,000.0
12 MONTHS OR LONGER	124,100,000	0
TOTAL	276,700,000	230,000,000.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	16,300,000	9,500,000
12 MONTHS OR LONGER	12,300,000	0
TOTAL	28,600,000	9,500,000
Corporate Debt
FAIR VALUE
LESS THAN 12 MONTHS	1,410,900,000	693,700,000
12 MONTHS OR LONGER	209,700,000	0
TOTAL	1,620,600,000	693,700,000
UNREALIZED LOSSES
LESS THAN 12 MONTHS	79,500,000	9,700,000
12 MONTHS OR LONGER	8,600,000	0
TOTAL	$ 88,100,000	9,700,000
Number of debt securities, available-for-sale | Security	5
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	$ 134,900,000
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	15,200,000
Net amount of recognized liabilities for repurchase agreements	800,000	0
Covered Bonds
FAIR VALUE
LESS THAN 12 MONTHS	233,600,000	92,100,000
12 MONTHS OR LONGER	73,400,000	0
TOTAL	307,000,000.0	92,100,000
UNREALIZED LOSSES
LESS THAN 12 MONTHS	14,500,000	900,000
12 MONTHS OR LONGER	1,500,000	0
TOTAL	16,000,000.0	900,000
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
LESS THAN 12 MONTHS	1,571,100,000	1,116,800,000
12 MONTHS OR LONGER	620,200,000	172,300,000
TOTAL	2,191,300,000	1,289,100,000
UNREALIZED LOSSES
LESS THAN 12 MONTHS	142,400,000	31,900,000
12 MONTHS OR LONGER	31,700,000	3,500,000
TOTAL	174,100,000	35,400,000
Other Asset-Backed
FAIR VALUE
LESS THAN 12 MONTHS	4,598,700,000	3,815,500,000
12 MONTHS OR LONGER	645,200,000	11,700,000
TOTAL	5,243,900,000	3,827,200,000
UNREALIZED LOSSES
LESS THAN 12 MONTHS	302,900,000	31,400,000
12 MONTHS OR LONGER	39,800,000	300,000
TOTAL	342,700,000	31,700,000
Commercial Mortgage-Backed
FAIR VALUE
LESS THAN 12 MONTHS	1,173,400,000	566,800,000
12 MONTHS OR LONGER	196,900,000	0
TOTAL	1,370,300,000	566,800,000
UNREALIZED LOSSES
LESS THAN 12 MONTHS	94,600,000	3,200,000
12 MONTHS OR LONGER	14,700,000	0
TOTAL	$ 109,300,000	$ 3,200,000