Investment Securities (Tables)

12 Months Ended

Dec. 31, 2019

Investments, Debt and Equity Securities [Abstract]

Amortized Cost and Approximate Fair Values of Investment Securities

The amortized cost and approximate fair values of investment securities are summarized as follows:

December 31, 2019	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale
U.S. Treasury notes	$	9.0	$	—	$	—		$	9.0
State, county and municipal securities	80.1		0.8		—			80.9
Obligations of U.S. government agencies	367.5		0.1		(0.8		)	366.8
U.S. agency residential mortgage-backed securities &    collateralized mortgage obligations	2,303.6		19.6		(6.0		)	2,317.2
Private mortgage-backed securities	47.6		—		(0.4		)	47.2
Corporate Securities	134.5		1.2		—			135.7
Other investments	3.2		—		—			3.2
Total	$	2,945.5	$	21.7	$	(7.2	)	$	2,960.0

December 31, 2019	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
Held-to Maturity
State, county and municipal securities	$	57.3	$	2.1	$	—	$	59.4
Obligations of U.S. government agencies	19.8		—		—		19.8
U.S agency residential mortgage-backed securities & collateralized mortgage obligations	1.2		—		—		1.2
Corporate securities	13.9		0.1		—		14.0
Other investments	0.1		—		—		0.1
Total	$	92.3	$	2.2	$	—	$	94.5

December 31, 2018	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale
U.S. Treasury notes	$	2.6	$	—	$	—		$	2.6
Obligations of U.S. government agencies	569.3		0.1		(10.2		)	559.2
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1,566.4		2.5		(24.1		)	1,544.8
Private mortgage-backed securities	72.0		—		(1.8		)	70.2
Corporate Securities	92.9		—		(1.0		)	91.9
Other investments	2.0		—		—			2.0
Total	$	2,305.2	$	2.6	$	(37.1	)	$	2,270.7

December 31, 2018	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to Maturity
State, county and municipal securities	$	150.9	$	1.8	$	(0.9	)	$	151.8
Obligations of U.S. government agencies	19.8		—		(0.3		)	19.5
U.S. agency residential mortgage-backed securities &    collateralized mortgage obligations	189.7		0.3		(6.5		)	183.5
Corporate securities	46.3		0.1		(0.6		)	45.8
Other investments	0.1		—		—			0.1
Total	$	406.8	$	2.2	$	(8.3	)	$	400.7

Gross Unrealized Losses and Fair Values of Investment Securities

The following tables show the gross unrealized losses and fair values of investment securities, aggregated by investment category, and the length of time individual investment securities have been in a continuous unrealized loss position, as of December 31, 2019 and 2018. There were no held-to-maturity securities in a continuous unrealized loss position as of December 31, 2019.

	Less than 12 Months					12 Months or More					Total
December 31, 2019	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale
Obligations of U.S. government agencies	$	185.3	$	(0.8	)	$	—	$	—		$	185.3	$	(0.8	)
U.S. agency residential mortgage-backed    securities & collateralized mortgage    obligations	740.1		(4.6		)	155.9		(1.4		)	896.0		(6.0		)
Private mortgage-backed securities	—		—			46.6		(0.4		)	46.6		(0.4		)
Total	$	925.4	$	(5.4	)	$	202.5	$	(1.8	)	$	1,127.9	$	(7.2	)

	Less than 12 Months					12 Months or More					Total
December 31, 2018	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale
Obligations of U.S. government agencies	$	363.1	$	(7.9	)	$	154.5	$	(2.3	)	$	517.6	$	(10.2	)
U.S. agency residential mortgage-backed    securities & collateralized mortgage    obligations	735.2		(14.5		)	503.7		(9.6		)	1,238.9		(24.1		)
Private mortgage-backed securities	—		—			69.4		(1.8		)	69.4		(1.8		)
Corporate securities	24.9		(0.2		)	51.4		(0.8		)	76.3		(1.0		)
Total	$	1,123.2	$	(22.6	)	$	779.0	$	(14.5	)	$	1,902.2	$	(37.1	)

	Less than 12 Months					12 Months or More					Total
December 31, 2018	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity
State, county and municipal securities	$	25.9	$	(0.3	)	$	57.1	$	(0.6	)	$	83.0	$	(0.9	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	45.0		(2.2		)	120.2		(4.3		)	165.2		(6.5		)
Corporate securities	—		—			39.6		(0.6		)	39.6		(0.6		)
Obligations of U.S. government agencies	19.5		(0.3		)	—		—			19.5		(0.3		)
Total	$	90.4	$	(2.8	)	$	216.9	$	(5.5	)	$	307.3	$	(8.3	)

Maturities of Investment Securities

All other investment securities maturities are shown at contractual maturity dates.

	Available-for-Sale				Held-to-Maturity
December 31, 2019	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Within one year	$	677.3	$	661.4	$	38.6	$	38.6
After one year but within five years	1,486.2		1,470.9		29.7		30.1
After five years but within ten years	328.0		428.2		21.3		23.1
After ten years	454.0		399.5		2.7		2.7
Total	$	2,945.5	$	2,960.0	$	92.3	$	94.5