Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Assets
Securities available for sale, at fair value	$ 4,873,778,000	$ 4,783,579,000
Mortgage loans held for sale	128,451,000	107,734,000
Mortgage loans held for investment, at fair value option	550,000	3,143,000
Derivative instruments	46,828,000	24,174,000
Liabilities
Derivative instruments	15,212,000	21,288,000
Fair value assets transferred from Level 1 to Level 2	0
Level 1
Assets
Securities available for sale, at fair value	0	0
Mortgage loans held for investment, at fair value option	0	0
Level 2
Assets
Securities available for sale, at fair value	4,873,778,000	4,783,579,000
Mortgage loans held for investment, at fair value option	0	0
Level 3
Assets
Securities available for sale, at fair value	0	0
Mortgage loans held for investment, at fair value option	550,000	3,143,000
Recurring
Assets
Securities available for sale, at fair value	4,873,778,000	4,783,579,000
Mortgage loans held for sale	128,451,000	107,734,000
Mortgage loans held for investment, at fair value option	550,000	3,143,000
Derivative instruments	51,086,000	27,048,000
Total, Assets	5,053,865,000	4,921,504,000
Liabilities
Derivative instruments	16,756,000	22,124,000
Total	16,756,000	22,124,000
Recurring \| Level 1
Assets
Securities available for sale, at fair value	0	0
Mortgage loans held for sale	0	0
Mortgage loans held for investment, at fair value option	0	0
Derivative instruments	0	0
Total, Assets	0	0
Liabilities
Derivative instruments	0	0
Total	0	0
Recurring \| Level 2
Assets
Securities available for sale, at fair value	4,873,778,000	4,783,579,000
Mortgage loans held for sale	128,451,000	107,734,000
Mortgage loans held for investment, at fair value option	0	0
Derivative instruments	51,086,000	27,048,000
Total, Assets	5,053,315,000	4,918,361,000
Liabilities
Derivative instruments	16,756,000	22,124,000
Total	16,756,000	22,124,000
Recurring \| Level 3
Assets
Securities available for sale, at fair value	0	0
Mortgage loans held for sale	0	0
Mortgage loans held for investment, at fair value option	550,000	3,143,000
Derivative instruments	0	0
Total, Assets	550,000	3,143,000
Liabilities
Derivative instruments	0	0
Total	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Fair value of financial and nonfinancial obligations.

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Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

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