Fair Value Measurements (Carrying Amounts and Estimated Fair Values) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investment securities:
Held-to-maturity	$ 618,672	$ 662,906
Financial liabilities:
Short-term borrowings	2,035,000	2,000,000
Long-term borrowings	426,002	372,882
Assets, fair value disclosure	3,216,470	2,098,201
Level 1
Financial liabilities:
Assets, fair value disclosure	0	0
Level 2
Financial liabilities:
Assets, fair value disclosure	3,145,457	2,034,360
Level 3
Financial liabilities:
Assets, fair value disclosure	71,013	63,841
Carrying Amount | Level 2
Investment securities:
Held-to-maturity	623,599	676,593
Financial liabilities:
Time deposits	4,074,248	3,217,207
Collateralized customer deposits	382,687	415,236
Short-term borrowings	2,035,000	2,000,000
Long-term borrowings	426,002	372,882
Carrying Amount | Level 3
Investment securities:
Held-to-maturity	500	500
Net loans	15,159,795	14,063,380
Fair Value | Level 2
Investment securities:
Held-to-maturity	618,232	662,516
Financial liabilities:
Time deposits	4,041,212	3,225,847
Collateralized customer deposits	382,370	415,236
Short-term borrowings	2,034,719	1,999,137
Long-term borrowings	423,258	367,984
Fair Value | Level 3
Investment securities:
Held-to-maturity	440	390
Net loans	$ 14,907,789	$ 14,114,545