Short-Term Borrowings (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) loan item	Dec. 31, 2017 USD ($)
Short-term Debt [Line Items]
Short-term borrowings	$ 1,589,128	$ 1,703,890
FHLB Borrowings [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 1,160,104	$ 1,132,567
FHLB Borrowings [Member] | FHLB Borrowings, 6 Fixed Rate Notes [Member] | Fixed-rate Term Notes [Member]
Short-term Debt [Line Items]
Number of fixed rate short-term borrowings | item	3
Short-term borrowings	$ 250,000
Short-term maturity year	2019
FHLB Borrowings [Member] | FHLB Borrowings, 6 Variable Rate Notes [Member] | Variable-rate Term Notes [Member]
Short-term Debt [Line Items]
Number of variable rate short-term borrowings | loan	6
Short-term borrowings	$ 910,000
FHLB Borrowings [Member] | FHLB Borrowings, 6 Variable Rate Notes [Member] | Debt Due 2020 [Member] | Variable-rate Term Notes [Member]
Short-term Debt [Line Items]
Short-term maturity year	2020
FHLB Borrowings [Member] | FHLB Borrowings, 6 Variable Rate Notes [Member] | Debt Due 2026 [Member] | Variable-rate Term Notes [Member]
Short-term Debt [Line Items]
Short-term maturity year	2026