Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Investment securities available for sale
Cost	$ 4,898,771	$ 4,607,117
Gross unrealized gains	15,594	6,425
Gross unrealized losses	9,482	56,432
Fair value	4,904,883	4,557,110
Cost	87,588	73,809
Gross unrealized gains	29,605	19,010
Gross unrealized losses	339	220
Fair value	116,854	92,599
Investment securities held to maturity
Cost	2,145,943	2,184,653
Gross unrealized gains	71,857
Gross unrealized losses	0
Fair value	2,217,800	2,201,502
Cost	7,132,302	6,865,579
Gross unrealized gains	117,056	42,774
Gross unrealized losses	9,821	57,142
Fair value	7,239,537	6,851,211
U.S. Treasury
Investment securities available for sale
Cost	748,206	1,249,243
Gross unrealized gains	1,114	633
Gross unrealized losses	2	2,166
Fair value	749,318	1,247,710
Government agency
Investment securities available for sale
Cost	666,029	257,252
Gross unrealized gains	1,404	222
Gross unrealized losses	1,812	639
Fair value	665,621	256,835
Mortgage-backed securities
Investment securities available for sale
Cost	3,329,593	2,956,793
Gross unrealized gains	10,369	5,309
Gross unrealized losses	7,343	52,763
Fair value	3,332,619	2,909,339
Investment securities held to maturity
Cost	2,115,141	2,184,653
Gross unrealized gains	71,857	17,339
Gross unrealized losses	0	490
Fair value	2,186,998	2,201,502
Corporate bonds
Investment securities available for sale
Cost	154,943	143,829
Gross unrealized gains	2,707	261
Gross unrealized losses	325	864
Fair value	157,325	$ 143,226
Other
Investment securities held to maturity
Cost	30,802
Gross unrealized gains	0
Gross unrealized losses	0
Fair value	$ 30,802

Debt Securities, Available-for-sale, Held-to-maturity and Equity Securities

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Debt Securities, Available-for-sale, Held-to-maturity and Equity Securities, Accumulated Gross Unrealized Gain, Before Tax

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount of unrealized loss on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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