Investment Securities - Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt securities available for sale:
Amortized Cost	$ 6,793,235	$ 6,639,277
Gross Unrealized Gains	240,033	91,083
Gross Unrealized Losses	8,044	10,359
Available-for-sale, at fair value (amortized cost of $6,793,235 and $6,639,277)	7,025,224	6,720,001
Debt securities held to maturity:
Amortized Cost	135,619	139,445
Gross Unrealized Gains	10,370	5,576
Gross Unrealized Losses	15	177
Fair Value	145,974	144,844
Government and government-sponsored enterprises
Debt securities available for sale:
Amortized Cost	218,996	235,045
Gross Unrealized Gains	80	18
Gross Unrealized Losses	496	678
Available-for-sale, at fair value (amortized cost of $6,793,235 and $6,639,277)	218,580	234,385
Obligations of states and political subdivisions
Debt securities available for sale:
Amortized Cost	850,816	852,096
Gross Unrealized Gains	16,509	12,446
Gross Unrealized Losses	769	687
Available-for-sale, at fair value (amortized cost of $6,793,235 and $6,639,277)	866,556	863,855
Total mortgage-backed debt securities
Debt securities available for sale:
Amortized Cost	5,722,972	5,551,685
Gross Unrealized Gains	223,444	78,619
Gross Unrealized Losses	6,728	8,973
Available-for-sale, at fair value (amortized cost of $6,793,235 and $6,639,277)	5,939,688	5,621,331
Residential agency
Debt securities available for sale:
Amortized Cost	4,701,950	4,492,427
Gross Unrealized Gains	194,932	68,797
Gross Unrealized Losses	13	6,103
Available-for-sale, at fair value (amortized cost of $6,793,235 and $6,639,277)	4,896,869	4,555,121
Debt securities held to maturity:
Amortized Cost	131,947	135,769
Gross Unrealized Gains	10,370	5,576
Gross Unrealized Losses	15	177
Fair Value	142,302	141,168
Residential non-agency
Debt securities available for sale:
Amortized Cost	327,663	374,046
Gross Unrealized Gains	415	1,166
Gross Unrealized Losses	4,544	616
Available-for-sale, at fair value (amortized cost of $6,793,235 and $6,639,277)	323,534	374,596
Commercial agency
Debt securities available for sale:
Amortized Cost	653,969	645,814
Gross Unrealized Gains	27,769	8,639
Gross Unrealized Losses	2,156	2,049
Available-for-sale, at fair value (amortized cost of $6,793,235 and $6,639,277)	679,582	652,404
Commercial non-agency
Debt securities available for sale:
Amortized Cost	39,390	39,398
Gross Unrealized Gains	328	17
Gross Unrealized Losses	15	205
Available-for-sale, at fair value (amortized cost of $6,793,235 and $6,639,277)	39,703	39,210
Corporate debt and trust preferred securities
Debt securities available for sale:
Amortized Cost	451	451
Gross Unrealized Gains	0	0
Gross Unrealized Losses	51	21
Available-for-sale, at fair value (amortized cost of $6,793,235 and $6,639,277)	400	430
Debt securities held to maturity:
Amortized Cost	3,672	3,676
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 3,672	$ 3,676

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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