Investment Securities (Contractual Maturities and Estimated Fair Values) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Held-to-maturity debt securities amortized cost disclosures
Due in one year or less, held-to-maturity debt securities amortized cost	$ 1,325,000		$ 1,325,000
Due after one year through five years, held-to-maturity debt securities amortized cost	1,075,000		1,075,000
Amortized cost, held-to-maturity debt securities	2,400,000		2,400,000		$ 2,400,000
Held-to-maturity debt securities, Estimated fair value disclosures
Due in one year or less, held-to-maturity debt securities, Estimated fair value	1,325,000		1,325,000
Due after one year through five years, held-to-maturity debt securities, Estimated fair value	1,075,000		1,075,000
Estimated fair value	2,400,000		2,400,000		2,400,000
Available-for-sale debt securities amortized cost disclosures
Due after five years through ten years, available-for-sale debt securities amortized cost	1,218,000		1,218,000
Due after ten years, available-for-sale debt securities amortized cost	244,208,000		244,208,000
Residential mortgage-backed securities, amortized cost	4,104,579,000		4,104,579,000
Equity securities, amortized cost	28,075,000		28,075,000
Amortized cost, Available for sale securities	4,378,080,000		4,378,080,000		4,196,034,000
Available for sale debt securities, Estimated Fair Value Disclosures
Due after five years through ten years, available-for-sale debt securities, Estimated Fair Value	1,307,000		1,307,000
Due after ten years, available-for-sale debt securities, Estimated Fair Value	260,944,000		260,944,000
Residential mortgage-backed securities, Estimated Fair Value	4,136,474,000		4,136,474,000
Equity securities, Estimated Fair Value	28,845,000		28,845,000
Estimated fair value, Available for sale securities	4,427,570,000		4,427,570,000		$ 4,199,372,000
Amortized cost of available for sale investment securities pledged	1,808,512,000		1,808,512,000
Fair value of available for sale investment securities pledged	1,827,783,000		1,827,783,000
Proceeds from sales and calls of available for sale securities	130,499,000	$ 0	326,037,000	$ 30,282,000
Proceeds from sales of mortgage-backed securities	117,713,000	0	311,931,000	23,992,000
Gross gains realized on sales	2,000	0	586,000	0
Gross losses realized on sales	$ 1,347,000	$ 0	$ 2,291,000	$ 428,000