Investment Securities Schedule of Amortized Cost and Fair Values of Debt Securities by Contractual Maturities (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Amortized Cost
Due in one year or less	$ 14,055
Due from one year to five years	36,871
Due from five years to ten years	119,069
Due after ten years	473,519
Amortized cost, before securities without debt maturities	643,514
Amortized Cost	2,655,314
Estimated Fair Value
Due in one year or less	14,063
Due from one year to five years	36,986
Due from five years to ten years	119,529
Due after ten years	461,487
Available for sale securities, debt maturities, before securities without single maturities	632,065
Estimated Fair Value	2,592,823
Collateralized Mortgage Obligations [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	695,840
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	679,700
Commercial Mortgage Backed Securities [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	231,378
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	226,892
Mortgage-Backed Securities [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	1,084,582
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	$ 1,054,166

Available for Sale Securities, Debt Maturities, Before Securities Without Single Maturities

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Available for Sale Securities, Debt Securities, Before Securities Without Debt Maturities

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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing after the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing after the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the next fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of available-for-sale debt securities at cost, net of adjustments, that do not have a single maturity date and which the entity has decided to disclose separately rather than allocating the cost over several maturity groupings. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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