Debt Securities Available for Sale and Debt Securities Held to Maturity - Adjusted cost and fair value of securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Debt securities available for sale:
Available-for-sale securities, amortized cost	$ 5,631,432		$ 7,251,523
Available-for-sale securities, gross unrealized gain	151,383		37,117
Available-for-sale securities, gross unrealized losses	17,623		53,335
Debt securities available for sale	5,765,192		7,235,305
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	8,708,454		6,797,046
Held-to-maturity securities, gross unrealized gain	387,598		151,345
Held-to-maturity securities, gross unrealized losses	17,754		27,233
Debt securities held to maturity, estimated fair value	9,078,298		6,921,158
Allowance for credit losses on debt securities held to maturity	15,017	$ 1,892	0
U.S. Treasury and other U.S. government agencies
Debt securities available for sale:
Available-for-sale securities, amortized cost	2,176,754		3,145,331
Available-for-sale securities, gross unrealized gain	56,764		16,888
Available-for-sale securities, gross unrealized losses	15,780		34,694
Debt securities available for sale	2,217,738		3,127,525
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	1,289,447		1,287,049
Held-to-maturity securities, gross unrealized gain	127,581		53,399
Held-to-maturity securities, gross unrealized losses	0		0
Debt securities held to maturity, estimated fair value	1,417,028		1,340,448
Agency mortgage-backed securities
Debt securities available for sale:
Available-for-sale securities, amortized cost	1,062,592		1,322,432
Available-for-sale securities, gross unrealized gain	35,382		12,444
Available-for-sale securities, gross unrealized losses	1,229		9,019
Debt securities available for sale	1,096,745		1,325,857
Agency collateralized mortgage obligations
Debt securities available for sale:
Available-for-sale securities, amortized cost	2,391,399		2,783,003
Available-for-sale securities, gross unrealized gain	59,210		7,744
Available-for-sale securities, gross unrealized losses	614		9,622
Debt securities available for sale	2,449,995		2,781,125
State and political subdivisions
Debt securities available for sale:
Available-for-sale securities, amortized cost	687		757
Available-for-sale securities, gross unrealized gain	27		41
Available-for-sale securities, gross unrealized losses	0		0
Debt securities available for sale	714		798
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	535,818		573,075
Held-to-maturity securities, gross unrealized gain	15,574		8,652
Held-to-maturity securities, gross unrealized losses	11,466		7,494
Debt securities held to maturity, estimated fair value	539,926		574,233
Allowance for credit losses on debt securities held to maturity	15,000
Agency
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	6,800,461		4,846,862
Held-to-maturity securities, gross unrealized gain	237,246		82,105
Held-to-maturity securities, gross unrealized losses	0		16,568
Debt securities held to maturity, estimated fair value	7,037,707		4,912,399
Non-agency
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	33,825		37,705
Held-to-maturity securities, gross unrealized gain	5,876		5,923
Held-to-maturity securities, gross unrealized losses	823		1,154
Debt securities held to maturity, estimated fair value	38,878		42,474
Asset-backed securities and other
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	48,903		52,355
Held-to-maturity securities, gross unrealized gain	1,321		1,266
Held-to-maturity securities, gross unrealized losses	5,465		2,017
Debt securities held to maturity, estimated fair value	$ 44,759		$ 51,604

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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Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity).

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