Investment Securities (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2016

Dec. 31, 2015

Investment Securities

Schedule of amortized cost and fair value of securities

	September 30, 2016								December 31, 2015
	Amortized		Unrealized		Unrealized		Fair		Amortized		Unrealized		Unrealized		Fair
(dollars in thousands)	Cost		Gains		Losses		Value		Cost		Gains		Losses		Value
U.S. Treasury securities	$	308,479	$	10	$	—	$	308,489	$	502,126	$	—	$	(2,150)	$	499,976
Government-sponsored enterprises debt securities		189,706		244		(57)		189,893		96,132		16		(324)		95,824
Government agency mortgage-backed securities		203,685		1,290		(17)		204,958		56,490		—		(508)		55,982
Government-sponsored enterprises mortgage-backed securities		8,539		461		—		9,000		10,185		560		—		10,745
Non-government mortgage-backed securities		—		—		—		—		—		157		—		157
Non-government asset-backed securities		19,510		—		(16)		19,494		95,453		—		(143)		95,310
Collateralized mortgage obligations:
Government agency		3,522,585		25,203		(4,136)		3,543,652		2,261,526		1,984		(23,576)		2,239,934
Government-sponsored enterprises		1,085,861		7,652		(5,303)		1,088,210		1,046,854		724		(18,241)		1,029,337
Total available-for-sale securities	$	5,338,365	$	34,860	$	(9,529)	$	5,363,696	$	4,068,766	$	3,441	$	(44,942)	$	4,027,265

	December 31,
	2015								2014
	Amortized		Unrealized		Unrealized		Fair		Amortized		Unrealized		Unrealized		Fair
(dollars in thousands)	Cost		Gains		Losses		Value		Cost		Gains		Losses		Value
U.S. Treasury securities	$	502,126	$	—	$	(2,150)	$	499,976	$	748,700	$	101	$	(286)	$	748,515
Non‑government securities		96,132		16		(324)		95,824		96,119		—		(547)		95,572
Government agency mortgage‑backed securities		56,490		—		(508)		55,982		—		—		—		—
Government‑sponsored enterprises mortgage‑backed securities		10,185		560		—		10,745		12,397		806		—		13,203
Non‑government mortgage‑backed securities		—		157		—		157		268		3,136		—		3,404
Non‑government asset‑backed securities		95,453		—		(143)		95,310		354,011		115		(134)		353,992
Collateralized mortgage obligations:
Government agency		2,261,526		1,984		(23,576)		2,239,934		2,699,632		8,567		(24,493)		2,683,706
Government‑sponsored enterprises		1,046,854		724		(18,241)		1,029,337		1,086,161		2,256		(19,414)		1,069,003
Equity securities		—		—		—		—		—		4,216		—		4,216
Total securities available for sale	$	4,068,766	$	3,441	$	(44,942)	$	4,027,265	$	4,997,288	$	19,197	$	(44,874)	$	4,971,611

Schedule of unrealized gross losses and fair values of securities in a continuous loss position

	Time in Continuous Loss as of September 30, 2016
	Less Than 12 Months				12 Months or More				Total
	Unrealized				Unrealized				Unrealized
(dollars in thousands)	Losses		Fair Value		Losses		Fair Value		Losses		Fair Value
Government-sponsored enterprises debt securities	$	(57)	$	24,931	$	—	$	—	$	(57)	$	24,931
Government agency mortgage-backed securities		(17)		26,287		—		—		(17)		26,287
Non-government asset-backed securities		—		609		(16)		11,680		(16)		12,289
Collateralized mortgage obligations:
Government agency		(1,102)		307,798		(3,034)		268,384		(4,136)		576,182
Government-sponsored enterprises		(82)		65,000		(5,221)		314,853		(5,303)		379,853
Total available-for-sale securities with unrealized losses	$	(1,258)	$	424,625	$	(8,271)	$	594,917	$	(9,529)	$	1,019,542

	Time in Continuous Loss as of December 31, 2015
	Less Than 12 Months				12 Months or More				Total
	Unrealized				Unrealized				Unrealized
(dollars in thousands)	Losses		Fair Value		Losses		Fair Value		Losses		Fair Value
U.S. Treasury securities	$	(2,150)	$	499,976	$	—	$	—	$	(2,150)	$	499,976
Government-sponsored enterprises debt securities		(324)		70,808		—		—		(324)		70,808
Government agency mortgage-backed securities		(508)		55,982		—		—		(508)		55,982
Non-government asset-backed securities		(143)		95,310		—		—		(143)		95,310
Collateralized mortgage obligations:
Government agency		(11,423)		1,428,423		(12,153)		354,335		(23,576)		1,782,758
Government-sponsored enterprises		(3,132)		532,122		(15,109)		354,987		(18,241)		887,109
Total available-for-sale securities with unrealized losses	$	(17,680)	$	2,682,621	$	(27,262)	$	709,322	$	(44,942)	$	3,391,943

	Time in Continuous Loss as of December 31, 2015
	Less Than 12 Months				12 Months or More				Total
	Unrealized		Fair		Unrealized		Fair		Unrealized		Fair
(dollars in thousands)	Losses		Value		Losses		Value		Losses		Value
U.S. Treasury securities	$	(2,150)	$	499,976	$	—	$	—	$	(2,150)	$	499,976
Non‑government securities		(324)		70,808		—		—		(324)		70,808
Government agency mortgage‑backed securities		(508)		55,982		—		—		(508)		55,982
Non‑government asset‑backed securities		(143)		95,310		—		—		(143)		95,310
Collateralized mortgage obligations:
Government agency		(11,423)		1,428,423		(12,153)		354,335		(23,576)		1,782,758
Government‑sponsored enterprises		(3,132)		532,122		(15,109)		354,987		(18,241)		887,109
Total securities available for sale with unrealized losses	$	(17,680)	$	2,682,621	$	(27,262)	$	709,322	$	(44,942)	$	3,391,943

	Time in Continuous Loss as of December 31, 2014
	Less Than 12 Months				12 Months or More				Total
	Unrealized		Fair		Unrealized		Fair		Unrealized		Fair
(dollars in thousands)	Losses		Value		Losses		Value		Losses		Value
U.S. Treasury securities	$	(286)	$	201,227	$	—	$	—	$	(286)	$	201,227
Non‑government securities		(143)		24,857		(404)		70,715		(547)		95,572
Non‑government asset‑backed securities		(134)		160,542		—		—		(134)		160,542
Collateralized mortgage obligations:
Government agency		(3,545)		497,235		(20,948)		670,377		(24,493)		1,167,612
Government‑sponsored enterprises		(964)		248,672		(18,450)		424,629		(19,414)		673,301
Total securities available for sale with unrealized losses	$	(5,072)	$	1,132,533	$	(39,802)	$	1,165,721	$	(44,874)	$	2,298,254

Schedule of amortized cost and fair value of securities by contractual maturity

	September 30, 2016
	Amortized		Fair
(dollars in thousands)	Cost		Value
Due after one year through five years	$	228,481	$	228,507
Due after five years through ten years		269,704		269,875
		498,185		498,382
Government agency mortgage-backed securities		203,685		204,958
Government-sponsored enterprises mortgage-backed securities		8,539		9,000
Non-government asset-backed securities		19,510		19,494
Collateralized mortgage obligations:
Government agency		3,522,585		3,543,652
Government-sponsored enterprises		1,085,861		1,088,210
Total mortgage- and asset-backed securities		4,840,180		4,865,314
Total available-for-sale securities	$	5,338,365	$	5,363,696

	December 31, 2015
	Amortized		Fair
(dollars in thousands)	Cost		Value
Due in one year or less	$	200,668	$	200,365
Due after one year through five years		397,590		395,435
Due after five years		—		—
		598,258		595,800
Government agency mortgage‑backed securities		56,490		55,982
Government‑sponsored enterprises mortgage‑backed securities		10,185		10,745
Non‑government mortgage‑backed securities		—		157
Non‑government asset‑backed securities		95,453		95,310
Collateralized mortgage obligations:
Government agency		2,261,526		2,239,934
Government‑sponsored enterprises		1,046,854		1,029,337
Total mortgage‑ and asset‑backed securities		3,470,508		3,431,465
Total securities available for sale	$	4,068,766	$	4,027,265