Short-Term Borrowings (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) loan item	Dec. 31, 2016 USD ($)
Short-term Debt [Line Items]
Short-term borrowings	$ 1,703,890	$ 1,225,406
FHLB Borrowings [Member]
Short-term Debt [Line Items]
Short-term borrowings	1,132,567	$ 865,000
FHLB Borrowings [Member] | $450 Debt Due 2020 [Member] | Variable-rate Term Notes [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 450,000
Short-term maturity year	2020
FHLB Borrowings [Member] | $200 Debt Due 2025 [Member] | Variable-rate Term Notes [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 200,000
Short-term maturity year	2025
FHLB Borrowings [Member] | $260 Debt Due 2026 [Member] | Variable-rate Term Notes [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 260,000
Short-term maturity year	2026
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	6
Short-term borrowings	$ 223,000
Short-term maturity year	2018
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