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Qualifying Debt (Details)	3 Months Ended		6 Months Ended
Qualifying Debt (Details)	Jun. 30, 2021 USD ($) $ / shares	Jun. 30, 2020 $ / shares	Jun. 30, 2021 USD ($) Trust	Dec. 31, 2020 USD ($)	Jun. 16, 2016 USD ($)	Jun. 29, 2015 USD ($)
Debt Instrument [Line Items]
Common Stock, Dividends, Per Share, Cash Paid \| $ / shares	$ 0.25	$ 0.25
Number of Statutory Business Trusts Formed or Acquired \| Trust			8
Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust	$ 79,600,000		$ 79,600,000	$ 78,900,000
Subordinated Long-term Debt, Noncurrent	1,000,000,000.0		1,000,000,000.0	469,800,000
Subordinated Debt [Member] \| Subordinated Debentures Maturing July 2056 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	175,000,000.0		175,000,000.0	175,000,000.0	$ 175,000,000.0
Debt Issuance Cost	$ 5,500,000		$ 5,500,000	$ 5,500,000
Debt Instrument, Interest Rate, Stated Percentage	6.25%		6.25%	6.25%
Subordinated Debt [Member] \| Subordinated Debentures Maturing July 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 75,000,000.0		$ 75,000,000.0	$ 75,000,000.0		$ 150,000,000.0
Debt Issuance Cost	$ 1,800,000		$ 1,800,000	$ 1,800,000
Debt Instrument, Interest Rate, Stated Percentage	3.38%		3.38%	3.44%
Subordinated Debt [Member] \| Subordinated Debentures Maturing June 2030 [Member] [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 225,000,000.0		$ 225,000,000.0	$ 225,000,000.0
Debt Issuance Cost	$ 3,100,000		$ 3,100,000	$ 3,100,000
Debt Instrument, Interest Rate, Stated Percentage	5.25%		5.25%	5.25%
Subordinated Debt [Member] \| Subordinated Debentures Maturing June 2031
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 600,000,000.0		$ 600,000,000.0
Debt Issuance Cost	$ 7,700,000		$ 7,700,000
Debt Instrument, Interest Rate, Stated Percentage	3.00%		3.00%
Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.48%		2.48%	2.58%