Investment Securities and Equity Securities with Readily Determinable Fair Values (Contractual Maturities and Estimated Fair Values) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Held-to-maturity debt securities amortized cost disclosures
Due in one year or less, held-to-maturity debt securities amortized cost	$ 1,200		$ 1,200
Due after one year through five years, held-to-maturity debt securities amortized cost	2,200		2,200
Amortized cost, held-to-maturity debt securities	3,400		3,400		$ 2,400
Held-to-maturity debt securities, Estimated fair value disclosures
Due in one year or less, held-to-maturity debt securities, Estimated fair value	1,200		1,200
Due after one year through five years, held-to-maturity debt securities, Estimated fair value	2,200		2,200
Estimated fair value	3,400		3,400		2,400
Available-for-sale debt securities amortized cost disclosures
Due after five years through ten years, available-for-sale debt securities amortized cost	355		355
Due after ten years, available-for-sale debt securities amortized cost	70,938		70,938
Residential mortgage-backed securities, amortized cost	2,973,842		2,973,842
Amortized cost, Available-for-sale securities	3,045,135		3,045,135		3,376,070
Available for sale debt securities, Estimated Fair Value Disclosures
Due after five years through ten years, available-for-sale debt securities, Estimated Fair Value	356		356
Due after ten years, available-for-sale debt securities, Estimated Fair Value	73,965		73,965
Residential mortgage-backed securities, Estimated Fair Value	3,023,413		3,023,413
Estimated fair value, Available for sale securities	3,097,734		3,097,734		$ 3,378,923
Proceeds from sales and calls of available for sale securities	0	$ 19,275	18,920	$ 60,695
Proceeds from sales of mortgage-backed securities	0	0	0	0
Gross gains realized on sales	0	1	0	3
Gross losses realized on sales	0	$ 7	5	$ 13
Collateral Pledged
Available-for-sale debt securities amortized cost disclosures
Amortized cost, Available-for-sale securities	1,151,578		1,151,578
Available for sale debt securities, Estimated Fair Value Disclosures
Fair value of available for sale investment securities pledged	$ 1,170,646		$ 1,170,646