Debt Securities Available for Sale and Debt Securities Held to Maturity - Adjusted cost and fair value of securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Debt securities available for sale:
Available-for-sale securities, amortized cost	$ 6,249,230		$ 7,251,523
Available-for-sale securities, gross unrealized gain	137,708		37,117
Available-for-sale securities, gross unrealized losses	42,122		53,335
Debt securities available for sale	6,344,816		7,235,305
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	7,878,158		6,797,046
Held-to-maturity securities, gross unrealized gain	403,289		151,345
Held-to-maturity securities, gross unrealized losses	14,054		27,233
Debt securities held to maturity, estimated fair value	8,267,393		6,921,158
Allowance for credit losses on debt securities held to maturity	1,892	$ 0	0
U.S. Treasury and other U.S. government agencies
Debt securities available for sale:
Available-for-sale securities, amortized cost	2,534,159		3,145,331
Available-for-sale securities, gross unrealized gain	62,833		16,888
Available-for-sale securities, gross unrealized losses	26,679		34,694
Debt securities available for sale	2,570,313		3,127,525
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	1,288,246		1,287,049
Held-to-maturity securities, gross unrealized gain	127,437		53,399
Held-to-maturity securities, gross unrealized losses	0		0
Debt securities held to maturity, estimated fair value	1,415,683		1,340,448
Agency mortgage-backed securities
Debt securities available for sale:
Available-for-sale securities, amortized cost	1,173,282		1,322,432
Available-for-sale securities, gross unrealized gain	24,195		12,444
Available-for-sale securities, gross unrealized losses	11,801		9,019
Debt securities available for sale	1,185,676		1,325,857
Agency collateralized mortgage obligations
Debt securities available for sale:
Available-for-sale securities, amortized cost	2,541,103		2,783,003
Available-for-sale securities, gross unrealized gain	50,649		7,744
Available-for-sale securities, gross unrealized losses	3,642		9,622
Debt securities available for sale	2,588,110		2,781,125
State and political subdivisions
Debt securities available for sale:
Available-for-sale securities, amortized cost	686		757
Available-for-sale securities, gross unrealized gain	31		41
Available-for-sale securities, gross unrealized losses	0		0
Debt securities available for sale	717		798
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	543,025		573,075
Held-to-maturity securities, gross unrealized gain	8,536		8,652
Held-to-maturity securities, gross unrealized losses	7,642		7,494
Debt securities held to maturity, estimated fair value	543,919		574,233
Allowance for credit losses on debt securities held to maturity	2,000
Agency
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	5,959,454		4,846,862
Held-to-maturity securities, gross unrealized gain	261,403		82,105
Held-to-maturity securities, gross unrealized losses	0		16,568
Debt securities held to maturity, estimated fair value	6,220,857		4,912,399
Non-agency
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	35,930		37,705
Held-to-maturity securities, gross unrealized gain	4,490		5,923
Held-to-maturity securities, gross unrealized losses	2,182		1,154
Debt securities held to maturity, estimated fair value	38,238		42,474
Asset-backed securities and other
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	51,503		52,355
Held-to-maturity securities, gross unrealized gain	1,423		1,266
Held-to-maturity securities, gross unrealized losses	4,230		2,017
Debt securities held to maturity, estimated fair value	$ 48,696		$ 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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