Investment Securities (Summary of Debt Securities Held-to-Maturity) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	$ 5,568,188	$ 5,293,918
Interest Receivable	57,800	59,000
Collateralized Mortgage Obligations [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	91,622	167,443
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	2,419,751	2,957,900
Agency commercial mortgage-backed securities (agency CMBS) [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	2,101,227	1,172,491
Municipal Bonds [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	739,507	740,431
Commercial Mortgage Backed Securities [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	216,081	$ 255,653
Moody's, Aaa Rating [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	425,457
Moody's, Aaa Rating [Member] | Collateralized Mortgage Obligations [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Aaa Rating [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Aaa Rating [Member] | Agency commercial mortgage-backed securities (agency CMBS) [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Aaa Rating [Member] | Municipal Bonds [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	209,376
Moody's, Aaa Rating [Member] | Commercial Mortgage Backed Securities [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	216,081
Moody's, Aa1 Rating [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	4,777,656
Moody's, Aa1 Rating [Member] | Collateralized Mortgage Obligations [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	91,622
Moody's, Aa1 Rating [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	2,419,751
Moody's, Aa1 Rating [Member] | Agency commercial mortgage-backed securities (agency CMBS) [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	2,101,227
Moody's, Aa1 Rating [Member] | Municipal Bonds [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	165,056
Moody's, Aa1 Rating [Member] | Commercial Mortgage Backed Securities [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Aa2 Rating [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	201,081
Moody's, Aa2 Rating [Member] | Collateralized Mortgage Obligations [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Aa2 Rating [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Aa2 Rating [Member] | Agency commercial mortgage-backed securities (agency CMBS) [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Aa2 Rating [Member] | Municipal Bonds [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	201,081
Moody's, Aa2 Rating [Member] | Commercial Mortgage Backed Securities [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Aa3 Rating [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	115,619
Moody's, Aa3 Rating [Member] | Collateralized Mortgage Obligations [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Aa3 Rating [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Aa3 Rating [Member] | Agency commercial mortgage-backed securities (agency CMBS) [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Aa3 Rating [Member] | Municipal Bonds [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	115,619
Moody's, Aa3 Rating [Member] | Commercial Mortgage Backed Securities [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, A1 Rating [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	33,264
Moody's, A1 Rating [Member] | Collateralized Mortgage Obligations [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, A1 Rating [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, A1 Rating [Member] | Agency commercial mortgage-backed securities (agency CMBS) [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, A1 Rating [Member] | Municipal Bonds [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	33,264
Moody's, A1 Rating [Member] | Commercial Mortgage Backed Securities [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, A2 Rating [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	8,475
Moody's, A2 Rating [Member] | Collateralized Mortgage Obligations [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, A2 Rating [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, A2 Rating [Member] | Agency commercial mortgage-backed securities (agency CMBS) [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, A2 Rating [Member] | Municipal Bonds [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	8,475
Moody's, A2 Rating [Member] | Commercial Mortgage Backed Securities [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, A3 Rating [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	2,066
Moody's, A3 Rating [Member] | Collateralized Mortgage Obligations [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, A3 Rating [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, A3 Rating [Member] | Agency commercial mortgage-backed securities (agency CMBS) [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, A3 Rating [Member] | Municipal Bonds [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	2,066
Moody's, A3 Rating [Member] | Commercial Mortgage Backed Securities [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Baa2 Rating [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	190
Moody's, Baa2 Rating [Member] | Collateralized Mortgage Obligations [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Baa2 Rating [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Baa2 Rating [Member] | Agency commercial mortgage-backed securities (agency CMBS) [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Baa2 Rating [Member] | Municipal Bonds [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	190
Moody's, Baa2 Rating [Member] | Commercial Mortgage Backed Securities [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Not Rated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	4,380
Moody's, Not Rated [Member] | Collateralized Mortgage Obligations [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Not Rated [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Not Rated [Member] | Agency commercial mortgage-backed securities (agency CMBS) [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	0
Moody's, Not Rated [Member] | Municipal Bonds [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	4,380
Moody's, Not Rated [Member] | Commercial Mortgage Backed Securities [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Investment securities held-to-maturity (fair value of $5,835,364 and $5,380,653)	$ 0