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	Mar. 31, 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	$ 628,943	$ 1,185,172
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	3,641	5,827
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	1,008,275	1,991,351
Available-for-sale securities, securities in a loss position for 12 months or longer, unrealized losses	38,481	47,508
Available-for-sale securities, fair value	1,637,218	3,176,523
Available-for-sale securities, unrealized losses	42,122	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	257,286	59,496
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	57	208
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	385,446	819,360
Available-for-sale securities, securities in a loss position for 12 months or longer, unrealized losses	26,622	34,486
Available-for-sale securities, fair value	642,732	878,856
Available-for-sale securities, unrealized losses	26,679	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	277,545	245,191
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	3,473	851
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	276,708	592,312
Available-for-sale securities, securities in a loss position for 12 months or longer, unrealized losses	8,328	8,168
Available-for-sale securities, fair value	554,253	837,503
Available-for-sale securities, unrealized losses	11,801	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	94,112	880,485
Available-for-sale securities, securities in a loss position for less than 12 months, unrealized losses	111	4,768
Available-for-sale securities, securities in a loss position for 12 months or longer, fair value	346,121	579,679
Available-for-sale securities, securities in a loss position for 12 months or longer, unrealized losses	3,531	4,854
Available-for-sale securities, fair value	440,233	1,460,164
Available-for-sale securities, unrealized losses	$ 3,642	$ 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.

+ References

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-Name Accounting Standards Codification
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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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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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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 unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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