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Investment Securities (Tables)

12 Months Ended

Dec. 31, 2017

Investment Securities

Amortized cost and estimated fair value by type of investment security

The amortized cost and estimated fair value by type of investment security at December 31, 2017 are as follows:

		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		$	3,943,092		$	14,110		$	(65,969)		$	3,891,233		$	3,891,233
Obligations of states and political subdivisions			225,096			7,871			(16)			232,951			232,951
Equity securities			28,075			293			(482)			27,886			27,886
Total investment securities		$	4,196,263		$	22,274		$	(66,467)		$	4,152,070		$	4,152,070

(1) Included in the carrying value of residential mortgage‑ backed securities are $654,063 of mortgage‑backed securities issued by Ginnie Mae and  $3,237,170 of mortgage‑backed securities issued by Fannie Mae and Freddie Mac

The amortized cost and estimated fair value by type of investment security at December 31, 2016 are as follows:

		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		$	3,946,144		$	18,246		$	(69,920)		$	3,894,470		$	3,894,470
Obligations of states and political subdivisions			244,622			10,783			(433)			254,972			254,972
Equity securities			28,075			314			(482)			27,907			27,907
Total investment securities		$	4,218,841		$	29,343		$	(70,835)		$	4,177,349		$	4,177,349

(1) Included in the carrying value of residential mortgage‑ backed securities are $850,033 of mortgage‑backed securities issued by Ginnie Mae, $3,029,832 of mortgage‑backed securities issued by Fannie Mae and Freddie Mac and $17,605 issued by non‑government entities

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		$	2,275		$	2,275		$	—		$	—
Due after one year through five years			125			125			—			—
Due after five years through ten years			—			—			1,939			1,967
Due after ten years			—			—			223,157			230,984
Residential mortgage-backed securities			—			—			3,943,092			3,891,233
Equity securities			—			—			28,075			27,886
Total investment securities		$	2,400		$	2,400		$	4,196,263		$	4,152,070

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, 2017 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		$	1,061,577		$	(13,157)		$	2,029,455		$	(52,812)		$	3,091,032		$	(65,969)
Obligations of states and political subdivisions			5,534			(9)			522			(7)			6,056			(16)
Equity securities			11,499			(251)			8,019			(231)			19,518			(482)
		$	1,078,610		$	(13,417)		$	2,037,996		$	(53,050)		$	3,116,606		$	(66,467)

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, 2016 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		$	2,513,872		$	(52,245)		$	396,695		$	(17,675)		$	2,910,567		$	(69,920)
Obligations of states and political subdivisions			31,104			(433)			—			—			31,104			(433)
Equity securities			14,066			(184)			5,452			(298)			19,518			(482)
		$	2,559,042		$	(52,862)		$	402,147		$	(17,973)		$	2,961,189		$	(70,835)

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, 2017 (in Thousands):

Balance at December 31, 2016		$	13,931
Sale of other-than-temporarily impaired available-for-sale securities during period			(13,931)
Balance at December 31, 2017		$	—

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, 2016 (in Thousands):

Balance at December 31, 2015		$	13,577
Impairment charges recognized during period			354
Balance at December 31, 2016		$	13,931

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

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