Fair Value Measurements - Changes in Level 3 Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) - Fair Value Measurements, Recurring [Member] - Significant Unobservable Inputs (Level 3) [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Privately Issued [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 28	$ 44
Total gains (losses) realized/unrealized:
Settlements	(1)	(3)
Ending Balance	27	41
Other Assets and Other Liabilities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	8,303	7,325
Total gains (losses) realized/unrealized:
Included in earnings	8,130	23,940
Transfers in and/or out of Level 3	(7,673)	(15,063)
Ending Balance	8,760	16,202
Changes in unrealized gains included in earnings related to assets still held at end of period	$ 8,778	$ 15,094