Borrowed Funds - Summary of Debt (Parenthetical) (Details)	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
4.875% Senior Notes, Due June 28, 2019
Debt Instrument [Line Items]
Debt instrument, interest rate	4.875%	4.875%
Debt instrument, maturity date	Jun. 28, 2019	Jun. 28, 2019
5.375% Senior Notes, Due June 28, 2021
Debt Instrument [Line Items]
Debt instrument, interest rate	5.375%	5.375%
Debt instrument, maturity date	Jun. 28, 2021	Jun. 28, 2021
7.250% Subordinated Notes, Due June 28, 2029, Callable in 2024
Debt Instrument [Line Items]
Debt instrument, interest rate	7.25%	7.25%
Debt instrument, maturity date	Jun. 28, 2029	Jun. 28, 2029
6.500% Subordinated Notes, Due March 2025, Callable in 2020
Debt Instrument [Line Items]
Debt instrument, interest rate	6.50%	6.50%
Debt instrument, maturity date	Mar. 31, 2025	Mar. 31, 2025
6.250% Subordinated Notes, Due June 28, 2029, Callable in 2024
Debt Instrument [Line Items]
Debt instrument, interest rate	6.25%	6.25%
Debt instrument, maturity date	Jun. 28, 2029	Jun. 28, 2029

Contractual interest rate for funds borrowed, under the debt agreement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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