Debt Securities Available for Sale and Debt Securities Held to Maturity - Adjusted cost and fair value of securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Debt securities available for sale:
Available-for-sale securities, amortized cost	$ 5,902,602		$ 7,251,523
Available-for-sale securities, gross unrealized gain	141,605		37,117
Available-for-sale securities, gross unrealized losses	16,135		53,335
Available-for-sale securities, estimated fair value	6,028,072		7,235,305
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	9,444,036		6,797,046
Held-to-maturity securities, gross unrealized gain	354,839		151,345
Held-to-maturity securities, gross unrealized losses	19,680		27,233
Debt securities held to maturity, estimated fair value	9,779,195		6,921,158
Allowance for debt securities held to maturity losses	15,105	$ 15,017	0
U.S. Treasury and other U.S. government agencies
Debt securities available for sale:
Available-for-sale securities, amortized cost	2,154,055		3,145,331
Available-for-sale securities, gross unrealized gain	51,784		16,888
Available-for-sale securities, gross unrealized losses	14,830		34,694
Available-for-sale securities, estimated fair value	2,191,009		3,127,525
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	1,290,667		1,287,049
Held-to-maturity securities, gross unrealized gain	121,608		53,399
Held-to-maturity securities, gross unrealized losses	0		0
Debt securities held to maturity, estimated fair value	1,412,275		1,340,448
Agency mortgage-backed securities
Debt securities available for sale:
Available-for-sale securities, amortized cost	935,845		1,322,432
Available-for-sale securities, gross unrealized gain	35,144		12,444
Available-for-sale securities, gross unrealized losses	1,067		9,019
Available-for-sale securities, estimated fair value	969,922		1,325,857
Agency collateralized mortgage obligations
Debt securities available for sale:
Available-for-sale securities, amortized cost	2,812,087		2,783,003
Available-for-sale securities, gross unrealized gain	54,656		7,744
Available-for-sale securities, gross unrealized losses	238		9,622
Available-for-sale securities, estimated fair value	2,866,505		2,781,125
State and political subdivisions
Debt securities available for sale:
Available-for-sale securities, amortized cost	615		757
Available-for-sale securities, gross unrealized gain	21		41
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities, estimated fair value	636		798
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	434,632		573,075
Held-to-maturity securities, gross unrealized gain	14,936		8,652
Held-to-maturity securities, gross unrealized losses	4,167		7,494
Debt securities held to maturity, estimated fair value	445,401		574,233
Allowance for debt securities held to maturity losses	15,000
Agency
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	7,638,230		4,846,862
Held-to-maturity securities, gross unrealized gain	211,081		82,105
Held-to-maturity securities, gross unrealized losses	10,399		16,568
Debt securities held to maturity, estimated fair value	7,838,912		4,912,399
Non-agency
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	31,760		37,705
Held-to-maturity securities, gross unrealized gain	5,946		5,923
Held-to-maturity securities, gross unrealized losses	435		1,154
Debt securities held to maturity, estimated fair value	37,271		42,474
Asset-backed securities and other
Debt securities held to maturity:
Held-to-maturity securities, amortized cost	48,747		52,355
Held-to-maturity securities, gross unrealized gain	1,268		1,266
Held-to-maturity securities, gross unrealized losses	4,679		2,017
Debt securities held to maturity, estimated fair value	$ 45,336		$ 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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Amortized cost of 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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