Debt Securities Available for Sale and Debt Securities Held to Maturity - Fair value and gross unrealized losses of available for sale debt securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	$ 158,800	$ 1,185,172
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	341	5,827
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	569,202	1,991,351
Available-for-sale securities, securities in a loss position for 12 months or longer, unrealized losses	15,794	47,508
Available-for-sale securities, fair value	728,002	3,176,523
Available-for-sale securities, unrealized losses	16,135	53,335
U.S. Treasury and other U.S. government agencies
Debt securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	5,226	59,496
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	67	208
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	351,472	819,360
Available-for-sale securities, securities in a loss position for 12 months or longer, unrealized losses	14,763	34,486
Available-for-sale securities, fair value	356,698	878,856
Available-for-sale securities, unrealized losses	14,830	34,694
Agency mortgage-backed securities
Debt securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	38,151	245,191
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	211	851
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	41,528	592,312
Available-for-sale securities, securities in a loss position for 12 months or longer, unrealized losses	856	8,168
Available-for-sale securities, fair value	79,679	837,503
Available-for-sale securities, unrealized losses	1,067	9,019
Agency collateralized mortgage obligations
Debt securities available for sale:
Available-for-sale securities, securities in a loss position for less than 12 months, fair value	115,423	880,485
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	63	4,768
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	176,202	579,679
Available-for-sale securities, securities in a loss position for 12 months or longer, unrealized losses	175	4,854
Available-for-sale securities, fair value	291,625	1,460,164
Available-for-sale securities, unrealized losses	$ 238	$ 9,622

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

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