Short-Term Borrowings (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)
Short-term Debt [Line Items]
Amount outstanding at period-end	$ 1,225,406	$ 1,423,644
FHLB Borrowings [Member]
Short-term Debt [Line Items]
Amount outstanding at period-end	865,000	$ 900,000
FHLB Borrowings [Member] | $415 Debt Due 2017 [Member]
Short-term Debt [Line Items]
Amount outstanding at period-end	$ 415,000
Short-term maturity year	2017
FHLB Borrowings [Member] | $450 Debt Due 2020 [Member]
Short-term Debt [Line Items]
Amount outstanding at period-end	$ 450,000
Short-term maturity year	2020
FHLB Borrowings [Member] | FHLB Borrowings, Notes # 1, 2, 3 [Member]
Short-term Debt [Line Items]
Amount outstanding at period-end	$ 225,000
Number of $225 million term note | item	3
FHLB Borrowings [Member] | FHLB Borrowings, Note # 4 [Member]
Short-term Debt [Line Items]
Amount outstanding at period-end	$ 190,000