Short-Term Borrowings (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) loan item	Dec. 31, 2018 USD ($)
Short-term Debt [Line Items]
Short-term borrowings	$ 2,714,872	$ 1,589,128
FHLB Borrowings [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 2,035,000	$ 1,160,104
FHLB Borrowings [Member] | FHLB Borrowings, 2 Notes, 1 Fixed and 1 Variable Rate Notes [Member] | Fixed-rate Term Notes [Member]
Short-term Debt [Line Items]
Number of fixed rate short-term borrowings | item	1
FHLB Borrowings [Member] | FHLB Borrowings, 2 Notes, 1 Fixed and 1 Variable Rate Notes [Member] | Fixed and Variable Rate Term Notes [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 775,000
Short-term maturity year	2020
FHLB Borrowings [Member] | FHLB Borrowings, 2 Notes, 1 Fixed and 1 Variable Rate Notes [Member] | Variable-rate Term Notes [Member]
Short-term Debt [Line Items]
Number of variable rate short-term borrowings | loan	1
FHLB Borrowings [Member] | FHLB Borrowings, 3 Fixed Rate Notes [Member] | Fixed-rate Term Notes [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 800,000
Number of fixed rate short-term borrowings | item	3
Short-term maturity year	2034
FHLB Borrowings [Member] | FHLB Borrowings, 4 Variable Rate Notes [Member] | Variable-rate Term Notes [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 460,000
Number of variable rate short-term borrowings | loan	4
FHLB Borrowings [Member] | FHLB Borrowings, 4 Variable Rate Notes [Member] | Variable-rate Term Notes [Member] | Debt Due 2025 [Member]
Short-term Debt [Line Items]
Short-term maturity year	2025
FHLB Borrowings [Member] | FHLB Borrowings, 4 Variable Rate Notes [Member] | Variable-rate Term Notes [Member] | Debt Due 2026 [Member]
Short-term Debt [Line Items]
Short-term maturity year	2026