Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Investment securities available for sale
Cost	$ 4,611,736	$ 4,607,117
Gross Unrealized Gains	9,387	6,425
Gross Unrealized Losses	31,323	56,432
Fair Value	4,589,800	4,557,110
Cost	79,809	73,809
Gross unrealized gains	30,252	19,010
Gross unrealized losses	177	220
Fair value	109,884	92,599
Investment securities held to maturity
Cost	2,214,829	2,184,653
Cost	6,906,374	6,865,579
Gross unrealized gains	80,184	42,774
Gross unrealized losses	31,950	57,142
Fair value	6,954,608	6,851,211
U. S. Treasury
Investment securities available for sale
Cost	1,196,584	1,249,243
Gross Unrealized Gains	1,305	633
Gross Unrealized Losses	847	2,166
Fair Value	1,197,042	1,247,710
Agency Securities
Investment securities available for sale
Cost	351,284	257,252
Gross Unrealized Gains	330	222
Gross Unrealized Losses	1,087	639
Fair Value	350,527	256,835
Mortgage Backed Securities
Investment securities available for sale
Cost	2,917,000	2,956,793
Gross Unrealized Gains	7,373	5,309
Gross Unrealized Losses	28,429	52,763
Fair Value	2,895,944	2,909,339
Investment securities held to maturity
Cost	2,214,829	2,184,653
Held To Maturity Securities Accumulated Unrecognized Holding Gain	40,545	17,339
Held To Maturity Securities Accumulated Unrecognized Holding Loss	450	490
Fair Value	2,254,924	2,201,502
Corporate Bonds
Investment securities available for sale
Cost	142,939	139,906
Gross Unrealized Gains	183	59
Gross Unrealized Losses	960	864
Fair Value	142,162	139,101
Other
Investment securities available for sale
Cost	3,929	3,923
Gross Unrealized Gains	196	202
Gross Unrealized Losses	0	0
Fair Value	$ 4,125	$ 4,125

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

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

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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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 investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

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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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