Investment Securities (Tables)

12 Months Ended

Dec. 31, 2015

Investment Securities

Amortized cost and estimated fair value by type of investment security

	Held to Maturity
			Gross		Gross
	Amortized		unrealized		unrealized		Estimated		Carrying
	cost		gains		losses		fair value		value
	(Dollars in Thousands)
Other securities	$	2,400	$	—	$	—	$	2,400	$	2,400
Total investment securities	$	2,400	$	—	$	—	$	2,400	$	2,400

	Available for Sale
			Gross		Gross
	Amortized		unrealized		unrealized		Estimated		Carrying
	cost		gains		losses		fair value		value(1)
	(Dollars in Thousands)
Residential mortgage-backed securities	$	3,908,809	$	30,959	$	(46,557)	$	3,893,211	$	3,893,211
Obligations of states and political subdivisions		259,150		18,579		(25)		277,704		277,704
Equity securities		28,075		627		(245)		28,457		28,457
Total investment securities	$	4,196,034	$	50,165	$	(46,827)	$	4,199,372	$	4,199,372

	Held to Maturity
			Gross		Gross
	Amortized		unrealized		unrealized		Estimated		Carrying
	cost		gains		losses		fair value		value
	(Dollars in Thousands)
Other securities	$	2,400	$	—	$	—	$	2,400	$	2,400
Total investment securities	$	2,400	$	—	$	—	$	2,400	$	2,400

	Available for Sale
			Gross		Gross		Estimated
	Amortized		unrealized		unrealized		fair		Carrying
	cost		gains		losses		value		value(1)
	(Dollars in Thousands)
Residential mortgage-backed securities	$	4,597,590	$	47,960	$	(45,178)	$	4,600,372	$	4,600,372
Obligations of states and political subdivisions		268,763		19,131		(5,618)		282,276		282,276
Equity securities		28,075		1,425		(185)		29,315		29,315
Total investment securities	$	4,894,428	$	68,516	$	(50,981)	$	4,911,963	$	4,911,963

Amortized cost and fair value of investment securities, by contractual maturity

	Held to Maturity				Available for Sale
	Amortized		Estimated		Amortized		Estimated
	Cost		fair value		Cost		fair value
	(Dollars in Thousands)
Due in one year or less	$	1,200	$	1,200	$	—	$	—
Due after one year through five years		1,200		1,200		—		—
Due after five years through ten years		—		—		—		—
Due after ten years		—		—		259,150		277,704
Residential mortgage-backed securities		—		—		3,908,809		3,893,211
Equity securities		—		—		28,075		28,457
Total investment securities	$	2,400	$	2,400	$	4,196,034	$	4,199,372

Gross unrealized losses on investment securities and the related fair value

Gross unrealized losses on investment securities and the fair value of the related securities, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, at December 31, 2015 were as follows:

	Less than 12 months				12 months or more				Total
			Unrealized				Unrealized				Unrealized
	Fair Value		Losses		Fair Value		Losses		Fair Value		Losses
	(Dollars in Thousands)
Available for sale:
Residential mortgage-backed securities	$	1,083,137	$	(9,333)	$	1,454,550	$	(37,224)	$	2,537,687	$	(46,557)
Obligations of states and political subdivisions		6,814		(19)		544		(6)		7,358		(25)
Equity securities		5,041		(35)		5,540		(210)		10,581		(245)
	$	1,094,992	$	(9,387)	$	1,460,634	$	(37,440)	$	2,555,626	$	(46,827)

Gross unrealized losses on investment securities and the fair value of the related securities, aggregated by investment category and length of time that individual securities have been in a continuous loss position, at December 31, 2014 were as follows:

	Less than 12 months				12 months or more				Total
			Unrealized				Unrealized				Unrealized
	Fair Value		Losses		Fair Value		Losses		Fair Value		Losses
	(Dollars in Thousands)
Available for sale:
Residential mortgage-backed securities	$	808,072	$	(4,910)	$	1,836,218	$	(40,268)	$	2,644,290	$	(45,178)
Obligations of states and political subdivisions		8,833		(97)		27,793		(5,521)		36,626		(5,618)
Equity securities		74		(1)		8,066		(184)		8,140		(185)
	$	816,979	$	(5,008)	$	1,872,077	$	(45,973)	$	2,689,056	$	(50,981)

Reconciliation of credit-related impairment charges on available-for-sale investment

The following table presents a reconciliation of credit‑related impairment charges on available‑for‑sale investments recognized in earnings for the twelve months ended December 31, 2015 (in Thousands):

Balance at December 31, 2014	$	12,623
Impairment charges recognized during period		954
Balance at December 31, 2015	$	13,577

The following table presents a reconciliation of credit‑related impairment charges on available‑for‑sale investments recognized in earnings for the twelve months ended December 31, 2014 (in Thousands):

Balance at December 31, 2013	$	11,806
Impairment charges recognized during period		817
Balance at December 31, 2014	$	12,623

The following table presents a reconciliation of credit‑related impairment charges on available‑for‑sale investments recognized in earnings for the twelve months ended December 31, 2013 (in Thousands):

Balance at December 31, 2012	$	10,432
Impairment charges recognized during period		1,374
Balance at December 31, 2013	$	11,806