Investment Securities and Equity Securities with Readily Determinable Fair Values (Contractual Maturities and Estimated Fair Values) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 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
Due after one year through five years, held-to-maturity debt securities amortized cost	1,200
Amortized cost, held-to-maturity debt securities	2,400		$ 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
Due after one year through five years, held-to-maturity debt securities, Estimated fair value	1,200
Estimated fair value	2,400		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
Due after ten years, available-for-sale debt securities amortized cost	71,040
Residential mortgage-backed securities, amortized cost	3,441,216
Amortized cost, Available-for-sale securities	3,512,611		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
Due after ten years, available-for-sale debt securities, Estimated Fair Value	73,826
Residential mortgage-backed securities, Estimated Fair Value	3,520,332
Estimated fair value, Available for sale securities	3,594,514		$ 3,378,923
Proceeds from sales and calls of available for sale securities	18,920	$ 41,420
Proceeds from sales of mortgage-backed securities	0	0
Gross gains realized on sales	0	2
Gross losses realized on sales	5	$ 6
Collateral Pledged
Available-for-sale debt securities amortized cost disclosures
Amortized cost, Available-for-sale securities	997,342
Available for sale debt securities, Estimated Fair Value Disclosures
Fair value of available for sale investment securities pledged	$ 1,020,890