Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers of financial assets between levels	$ 0
Fair Value, Measurements, Recurring
Assets:
Assets, fair value	2,284,398,000	$ 2,132,636,000
Liabilities:
Liabilities, fair value	16,839,000	11,833,000
Fair Value, Measurements, Recurring | Derivative liabilities - Interest rate contracts | Interest Rate Swap
Liabilities:
Liabilities, fair value	16,839,000	11,833,000
Fair Value, Measurements, Recurring | Level 1
Assets:
Assets, fair value	69,075,000	68,924,000
Liabilities:
Liabilities, fair value	0	0
Fair Value, Measurements, Recurring | Level 1 | Derivative liabilities - Interest rate contracts | Interest Rate Swap
Liabilities:
Liabilities, fair value	0	0
Fair Value, Measurements, Recurring | Level 2
Assets:
Assets, fair value	2,215,323,000	2,063,712,000
Liabilities:
Liabilities, fair value	16,839,000	11,833,000
Fair Value, Measurements, Recurring | Level 2 | Derivative liabilities - Interest rate contracts | Interest Rate Swap
Liabilities:
Liabilities, fair value	16,839,000	11,833,000
Fair Value, Measurements, Recurring | Level 3
Assets:
Assets, fair value	0	0
Liabilities:
Liabilities, fair value	0	0
Fair Value, Measurements, Recurring | Level 3 | Derivative liabilities - Interest rate contracts | Interest Rate Swap
Liabilities:
Liabilities, fair value	0	0
Fair Value, Measurements, Recurring | U.S. Government agencies and sponsored enterprises obligations
Assets:
Assets, fair value	90,010,000	42,838,000
Fair Value, Measurements, Recurring | U.S. Government agencies and sponsored enterprises obligations | Level 1
Assets:
Assets, fair value	0	0
Fair Value, Measurements, Recurring | U.S. Government agencies and sponsored enterprises obligations | Level 2
Assets:
Assets, fair value	90,010,000	42,838,000
Fair Value, Measurements, Recurring | U.S. Government agencies and sponsored enterprises obligations | Level 3
Assets:
Assets, fair value	0	0
Fair Value, Measurements, Recurring | U.S. Government agencies and sponsored enterprises mortgage-backed securities
Assets:
Assets, fair value	596,436,000	595,237,000
Fair Value, Measurements, Recurring | U.S. Government agencies and sponsored enterprises mortgage-backed securities | Level 1
Assets:
Assets, fair value	0	0
Fair Value, Measurements, Recurring | U.S. Government agencies and sponsored enterprises mortgage-backed securities | Level 2
Assets:
Assets, fair value	596,436,000	595,237,000
Fair Value, Measurements, Recurring | U.S. Government agencies and sponsored enterprises mortgage-backed securities | Level 3
Assets:
Assets, fair value	0	0
Fair Value, Measurements, Recurring | State and municipal obligations
Assets:
Assets, fair value	25,036,000	26,172,000
Fair Value, Measurements, Recurring | State and municipal obligations | Level 1
Assets:
Assets, fair value	0	0
Fair Value, Measurements, Recurring | State and municipal obligations | Level 2
Assets:
Assets, fair value	25,036,000	26,172,000
Fair Value, Measurements, Recurring | State and municipal obligations | Level 3
Assets:
Assets, fair value	0	0
Fair Value, Measurements, Recurring | Asset-backed securities
Assets:
Assets, fair value	671,609,000	610,546,000
Fair Value, Measurements, Recurring | Asset-backed securities | Level 1
Assets:
Assets, fair value	0	0
Fair Value, Measurements, Recurring | Asset-backed securities | Level 2
Assets:
Assets, fair value	671,609,000	610,546,000
Fair Value, Measurements, Recurring | Asset-backed securities | Level 3
Assets:
Assets, fair value	0	0
Fair Value, Measurements, Recurring | Corporate bonds and other debt securities
Assets:
Assets, fair value	797,479,000	755,903,000
Fair Value, Measurements, Recurring | Corporate bonds and other debt securities | Level 1
Assets:
Assets, fair value	56,321,000	55,970,000
Fair Value, Measurements, Recurring | Corporate bonds and other debt securities | Level 2
Assets:
Assets, fair value	741,158,000	699,933,000
Fair Value, Measurements, Recurring | Corporate bonds and other debt securities | Level 3
Assets:
Assets, fair value	0	0
Fair Value, Measurements, Recurring | Preferred stock and other equity securities
Assets:
Assets, fair value	88,476,000	90,107,000
Fair Value, Measurements, Recurring | Preferred stock and other equity securities | Level 1
Assets:
Assets, fair value	12,754,000	12,954,000
Fair Value, Measurements, Recurring | Preferred stock and other equity securities | Level 2
Assets:
Assets, fair value	75,722,000	77,153,000
Fair Value, Measurements, Recurring | Preferred stock and other equity securities | Level 3
Assets:
Assets, fair value	0	0
Fair Value, Measurements, Recurring | Derivative assets - Interest rate contracts | Interest Rate Swap
Assets:
Assets, fair value	15,352,000	11,833,000
Fair Value, Measurements, Recurring | Derivative assets - Interest rate contracts | Level 1 | Interest Rate Swap
Assets:
Assets, fair value	0	0
Fair Value, Measurements, Recurring | Derivative assets - Interest rate contracts | Level 2 | Interest Rate Swap
Assets:
Assets, fair value	15,352,000	11,833,000
Fair Value, Measurements, Recurring | Derivative assets - Interest rate contracts | Level 3 | Interest Rate Swap
Assets:
Assets, fair value	$ 0	$ 0