Securities (Securities with Unrealized Losses) (Details)	Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($) loan
FAIR VALUE
Less Than 12 Months, Fair Value	$ 2,891,700,000	$ 8,748,500,000
12 Months or Longer, Fair Value	8,781,200,000	9,083,700,000
Total, Fair Value	11,672,900,000	17,832,200,000
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	5,600,000	28,300,000
12 Months or Longer, Unrealized Losses	153,100,000	226,800,000
Total, Unrealized Losses	158,700,000	255,100,000
Allowance for credit losses, available-for-sale securities	0
U.S. Government
FAIR VALUE
Less Than 12 Months, Fair Value	0	4,477,500,000
12 Months or Longer, Fair Value	194,200,000	532,300,000
Total, Fair Value	194,200,000	5,009,800,000
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0	11,100,000
12 Months or Longer, Unrealized Losses	5,100,000	11,800,000
Total, Unrealized Losses	5,100,000	22,900,000
Obligations of States and Political Subdivisions
FAIR VALUE
Less Than 12 Months, Fair Value	0	0
12 Months or Longer, Fair Value	313,100,000	297,500,000
Total, Fair Value	313,100,000	297,500,000
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0	0
12 Months or Longer, Unrealized Losses	9,300,000	13,600,000
Total, Unrealized Losses	9,300,000	13,600,000
Government Sponsored Agency
FAIR VALUE
Less Than 12 Months, Fair Value	1,288,200,000	3,298,800,000
12 Months or Longer, Fair Value	6,848,500,000	6,373,800,000
Total, Fair Value	8,136,700,000	9,672,600,000
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	1,700,000	14,500,000
12 Months or Longer, Unrealized Losses	91,600,000	118,000,000.0
Total, Unrealized Losses	93,300,000	132,500,000
Non-U.S. Government
FAIR VALUE
Less Than 12 Months, Fair Value	329,700,000	54,900,000
12 Months or Longer, Fair Value	197,500,000	181,600,000
Total, Fair Value	527,200,000	236,500,000
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	100,000	100,000
12 Months or Longer, Unrealized Losses	6,800,000	12,200,000
Total, Unrealized Losses	6,900,000	12,300,000
Corporate Debt
FAIR VALUE
Less Than 12 Months, Fair Value	21,300,000	0
12 Months or Longer, Fair Value	43,100,000	76,900,000
Total, Fair Value	64,400,000	76,900,000
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	400,000	0
12 Months or Longer, Unrealized Losses	300,000	1,600,000
Total, Unrealized Losses	700,000	$ 1,600,000
Number of debt securities excluded, available-for-sale | loan		1
Debt securities, available-for-sale, continuous unrealized loss position with credit loss		$ 38,900,000
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss		1,300,000
Allowance for credit losses, available-for-sale securities		200,000
Covered Bonds
FAIR VALUE
Less Than 12 Months, Fair Value	80,000,000.0	0
12 Months or Longer, Fair Value	63,700,000	119,600,000
Total, Fair Value	143,700,000	119,600,000
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	1,000,000.0	0
12 Months or Longer, Unrealized Losses	1,200,000	3,600,000
Total, Unrealized Losses	2,200,000	3,600,000
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
Less Than 12 Months, Fair Value	1,007,700,000	881,900,000
12 Months or Longer, Fair Value	669,100,000	720,500,000
Total, Fair Value	1,676,800,000	1,602,400,000
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	2,300,000	2,600,000
12 Months or Longer, Unrealized Losses	27,300,000	39,600,000
Total, Unrealized Losses	29,600,000	42,200,000
Other Asset-Backed
FAIR VALUE
Less Than 12 Months, Fair Value	109,900,000	35,400,000
12 Months or Longer, Fair Value	265,400,000	405,400,000
Total, Fair Value	375,300,000	440,800,000
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	100,000	0
12 Months or Longer, Unrealized Losses	3,000,000.0	12,900,000
Total, Unrealized Losses	3,100,000	12,900,000
Commercial Mortgage-Backed
FAIR VALUE
Less Than 12 Months, Fair Value	54,900,000	0
12 Months or Longer, Fair Value	186,600,000	376,100,000
Total, Fair Value	241,500,000	376,100,000
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0	0
12 Months or Longer, Unrealized Losses	8,500,000	13,500,000
Total, Unrealized Losses	$ 8,500,000	$ 13,500,000