Acquisitions - Assets Acquired and Liabilities Assumed - (Details) - USD ($) $ in Thousands	Jun. 01, 2020	May 29, 2020	Jul. 01, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract]
Goodwill				$ 898,569	$ 808,322	$ 808,726		$ 493,329
Opus
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents	$ 937,102
Interest bearing time deposits with financial institutions	137
Investment securities	829,891
Loans	5,809,451
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Allowance for Loan Losses	(21,242)
Fixed assets	22,121
Intangible assets/Core deposit intangible	19,267
Deferred tax assets	45,003
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets	370,304
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets, Total	8,012,034
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits	6,915,990
FHLB advances and other borrowings	213,491
Subordinated debt/Borrowings	138,653
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other	84,542
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities, Total	7,352,676
Total fair value of identifiable net assets	659,358
Total merger consideration	749,605	$ 749,605
Goodwill	$ 90,247
Grandpoint Capital, Inc.
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents			$ 147,551
Investment securities			392,858
Loans			2,352,717
Fixed assets			9,122
Intangible assets/Core deposit intangible			71,943
Deferred tax assets			5,028
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets			97,005
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets, Total			3,076,224				$ 3,050,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits			2,506,929				$ 2,510,000
Subordinated debt/Borrowings			254,923
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other			24,859
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities, Total			2,786,711
Total fair value of identifiable net assets			289,513
Total merger consideration			602,152
Goodwill			$ 312,639