Investment Securities and Equity Securities with Readily Determinable Fair Values (Contractual Maturities and Estimated Fair Values) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Held-to-maturity debt securities amortized cost disclosures
Due in one year or less, held-to-maturity debt securities amortized cost	$ 1,075		$ 1,075
Due after one year through five years, held-to-maturity debt securities amortized cost	1,325		1,325
Amortized cost, held-to-maturity debt securities	2,400		2,400		$ 1,200
Held-to-maturity debt securities, Estimated fair value disclosures
Due in one year or less, held-to-maturity debt securities, Estimated fair value	1,075		1,075
Due after one year through five years, held-to-maturity debt securities, Estimated fair value	1,325		1,325
Estimated fair value	2,400		2,400		1,200
Available-for-sale debt securities amortized cost disclosures
Due after five years through ten years, available-for-sale debt securities amortized cost	2,243		2,243
Due after ten years, available-for-sale debt securities amortized cost	89,956		89,956
Residential mortgage-backed securities, amortized cost	3,095,049		3,095,049
Amortized cost, Available-for-sale securities	3,187,248		3,187,248		3,481,165
Available for sale debt securities, Estimated Fair Value Disclosures
Due after five years through ten years, available-for-sale debt securities, Estimated Fair Value	2,267		2,267
Due after ten years, available-for-sale debt securities, Estimated Fair Value	93,195		93,195
Residential mortgage-backed securities, Estimated Fair Value	3,097,410		3,097,410
Estimated fair value, Available for sale securities	3,192,872		3,192,872		$ 3,411,350
Proceeds from sales and calls of available for sale securities	32,285	$ 19,530	92,980	$ 37,675
Proceeds from sales of mortgage-backed securities	0	0	0	0
Gross gains realized on sales	1	2	4	3
Gross losses realized on sales	3	$ 0	16	$ 0
Collateral Pledged
Available-for-sale debt securities amortized cost disclosures
Amortized cost, Available-for-sale securities	1,143,623		1,143,623
Available for sale debt securities, Estimated Fair Value Disclosures
Fair value of available for sale investment securities pledged	$ 1,142,310		$ 1,142,310